diff --git a/README.md b/README.md
index efbdc3c2..a6f427dd 100644
--- a/README.md
+++ b/README.md
@@ -1918,6 +1918,11 @@ polymorfa webhook create \
   --input webhook.json \
   --secret-out ./webhook.secret
 
+polymorfa webhook update webhook_123 \
+  --owner project \
+  --project project_123 \
+  --phone-offline-start-day 8
+
 polymorfa webhook test webhook_123 \
   --owner project \
   --project project_123 \
@@ -1933,6 +1938,14 @@ polymorfa webhook delivery attempt list delivery_123 \
   --project project_123
 ```
 
+`--phone-offline-start-day` on `webhook create` and `webhook update` sets the
+first day without the phone, from `7` to `13`, on which the webhook receives
+`session.phone_offline`. The API default is `10`. `off` removes that event from
+the webhook's event types; `webhook update` reads the saved event types when
+`--input` has none, and refuses `off` when it would leave no event. `webhook
+update` accepts the flag without `--input`. See
+[Phone offline warnings](https://docs.polymorfa.com/api/webhooks#phone-offline-warnings).
+
 `webhook test` requires `--event`. Team tests accept that event type only.
 Project tests can also read a JSON object with `body` and `sessionId` from
 `--input`; supply both fields together. The CLI rejects a team body or an
diff --git a/src/commands/webhook/create.ts b/src/commands/webhook/create.ts
index b2ff9642..bcca62f2 100644
--- a/src/commands/webhook/create.ts
+++ b/src/commands/webhook/create.ts
@@ -9,6 +9,10 @@ import {prepareOneTimeSecretDelivery} from '../../developer/secrets.js'
 import {record, stringField} from '../../developer/support.js'
 import {CliConditionError} from '../../errors/conditions.js'
 import {idempotencyKey} from '../../input/idempotency.js'
+import {
+  phoneOfflineStartDayFlag,
+  withPhoneOfflineStartDay,
+} from '../../developer/phone-offline.js'
 
 export default class WebhookCreate extends BaseCommand {
   static override description = 'Create a webhook from one JSON object.'
@@ -19,6 +23,7 @@ export default class WebhookCreate extends BaseCommand {
       description: 'Read the webhook JSON object from a file or stdin (-).',
       required: true,
     }),
+    'phone-offline-start-day': phoneOfflineStartDayFlag,
     'secret-out': Flags.string({
       description: 'Write the one-time signing secret to a new mode-0600 file.',
     }),
@@ -30,11 +35,20 @@ export default class WebhookCreate extends BaseCommand {
       json: flags.json === true,
       ...(flags['secret-out'] === undefined ? {} : {path: flags['secret-out']}),
     })
+    const body = record(await readJsonInput(flags.input), 'Webhook input')
+    let merged: Readonly<Record<string, unknown>> = body
+    if (flags['phone-offline-start-day'] !== undefined) {
+      try {
+        merged = withPhoneOfflineStartDay(
+          body,
+          flags['phone-offline-start-day'],
+        )
+      } catch (error) {
+        this.error((error as Error).message, {exit: 2})
+      }
+    }
     const session = await this.developerSession(flags)
-    const input = record(
-      await readJsonInput(flags.input),
-      'Webhook input',
-    ) as unknown as CreateProjectWebhookInput
+    const input = merged as unknown as CreateProjectWebhookInput
     const response = await session.client.webhooks.create(input, {
       ...session.requestOptions,
       idempotencyKey: idempotencyKey(flags['idempotency-key']),
diff --git a/src/commands/webhook/update.ts b/src/commands/webhook/update.ts
index 3bfb9b46..4d4f6de7 100644
--- a/src/commands/webhook/update.ts
+++ b/src/commands/webhook/update.ts
@@ -6,6 +6,10 @@ import {idempotencyFlag} from '../../developer/flags.js'
 import {developerMutation} from '../../developer/output.js'
 import {record, stringField} from '../../developer/support.js'
 import {idempotencyKey} from '../../input/idempotency.js'
+import {
+  phoneOfflineStartDayFlag,
+  withPhoneOfflineStartDay,
+} from '../../developer/phone-offline.js'
 
 export default class WebhookUpdate extends BaseCommand {
   static override args = {
@@ -18,22 +22,43 @@ export default class WebhookUpdate extends BaseCommand {
     'idempotency-key': idempotencyFlag,
     input: Flags.string({
       description:
-        'Read the webhook update JSON object from a file or stdin (-).',
-      required: true,
+        'Read the webhook update JSON object from a file or stdin (-). Optional with --phone-offline-start-day.',
     }),
+    'phone-offline-start-day': phoneOfflineStartDayFlag,
   }
 
   async run(): Promise<void> {
     const {args, flags} = await this.parse(WebhookUpdate)
+    const startDay = flags['phone-offline-start-day']
+    if (flags.input === undefined && startDay === undefined) {
+      this.error('Provide --input or --phone-offline-start-day.', {exit: 2})
+    }
+    let body: Readonly<Record<string, unknown>> =
+      flags.input === undefined
+        ? {}
+        : record(await readJsonInput(flags.input), 'Webhook input')
     const session = await this.developerSession(flags)
-    const response = await session.client.webhooks.update(
-      args.webhook,
-      record(await readJsonInput(flags.input), 'Webhook input'),
-      {
-        ...session.requestOptions,
-        idempotencyKey: idempotencyKey(flags['idempotency-key']),
-      },
-    )
+    if (startDay !== undefined) {
+      // Off edits the saved event list unless the input already supplies one.
+      const saved =
+        startDay === 'off' && !Array.isArray(body.eventTypes)
+          ? (
+              await session.client.webhooks.retrieve(
+                args.webhook,
+                session.requestOptions,
+              )
+            ).data.eventTypes
+          : undefined
+      try {
+        body = withPhoneOfflineStartDay(body, startDay, saved)
+      } catch (error) {
+        this.error((error as Error).message, {exit: 2})
+      }
+    }
+    const response = await session.client.webhooks.update(args.webhook, body, {
+      ...session.requestOptions,
+      idempotencyKey: idempotencyKey(flags['idempotency-key']),
+    })
     const receipt = record(response.data, 'Webhook mutation receipt')
     const webhook = record(receipt.webhook, 'Webhook')
     const output = developerMutation({
diff --git a/src/developer/phone-offline.ts b/src/developer/phone-offline.ts
new file mode 100644
index 00000000..1e7ca3c3
--- /dev/null
+++ b/src/developer/phone-offline.ts
@@ -0,0 +1,48 @@
+import {Flags} from '@oclif/core'
+
+export const PHONE_OFFLINE_EVENT = 'session.phone_offline'
+const MIN_DAY = 7
+const MAX_DAY = 13
+
+/** A day from 7 to 13, or off to unsubscribe from the warning. */
+export type PhoneOfflineStartDay = number | 'off'
+
+function parseStartDay(value: string): PhoneOfflineStartDay {
+  if (value === 'off') return 'off'
+  const day = Number(value)
+  if (!/^\d+$/.test(value) || day < MIN_DAY || day > MAX_DAY) {
+    throw new Error(`Expected a day from ${MIN_DAY} to ${MAX_DAY}, or off.`)
+  }
+  return day
+}
+
+export const phoneOfflineStartDayFlag = Flags.custom<PhoneOfflineStartDay>({
+  description: `First day without the phone on which the webhook receives ${PHONE_OFFLINE_EVENT} (${MIN_DAY}-${MAX_DAY}), or off to remove that event. WhatsApp signs the device out on day 14.`,
+  parse: (value) => Promise.resolve(parseStartDay(value)),
+})()
+
+/**
+ * Merge the flag into a webhook body. A day sets phoneOfflineStartDay; off
+ * removes the event from eventTypes, read from the body or else the saved
+ * webhook. Platform webhooks need at least one event, so off cannot empty it.
+ */
+export function withPhoneOfflineStartDay(
+  body: Readonly<Record<string, unknown>>,
+  value: PhoneOfflineStartDay,
+  savedEventTypes?: readonly string[],
+): Record<string, unknown> {
+  if (value !== 'off') return {...body, phoneOfflineStartDay: value}
+  const eventTypes = Array.isArray(body.eventTypes)
+    ? (body.eventTypes as unknown[]).map(String)
+    : savedEventTypes
+  if (eventTypes === undefined) {
+    throw new Error('Add eventTypes to the input before using off.')
+  }
+  const remaining = eventTypes.filter((event) => event !== PHONE_OFFLINE_EVENT)
+  if (remaining.length === 0) {
+    throw new Error(
+      `A webhook needs at least one event besides ${PHONE_OFFLINE_EVENT}. Delete or disable the webhook instead.`,
+    )
+  }
+  return {...body, eventTypes: remaining}
+}
diff --git a/src/developer/render.ts b/src/developer/render.ts
index c3413261..e1526fac 100644
--- a/src/developer/render.ts
+++ b/src/developer/render.ts
@@ -35,6 +35,17 @@ export function renderEventDetail(
   return table(rows)
 }
 
+/** Off when the explicit event list leaves the warning out. */
+function phoneOfflineWarning(webhook: {
+  readonly eventTypes: readonly string[]
+  readonly phoneOfflineStartDay?: number
+}): string {
+  if (!webhook.eventTypes.includes('session.phone_offline')) return 'off'
+  return webhook.phoneOfflineStartDay === undefined
+    ? 'not reported'
+    : `from day ${webhook.phoneOfflineStartDay}`
+}
+
 export function renderWebhookDetail(webhook: {
   readonly id: string
   readonly owner: string
@@ -44,6 +55,7 @@ export function renderWebhookDetail(webhook: {
   readonly format: string
   readonly eventTypes: readonly string[]
   readonly headers: readonly {readonly name: string}[]
+  readonly phoneOfflineStartDay?: number
   readonly createdAt: string
   readonly updatedAt: string
 }): string {
@@ -55,6 +67,7 @@ export function renderWebhookDetail(webhook: {
     ['Enabled', webhook.enabled],
     ['Format', webhook.format],
     ['Event types', webhook.eventTypes.join(', ')],
+    ['Phone offline warning', phoneOfflineWarning(webhook)],
     ['Header names', webhook.headers.map(({name}) => name).join(', ')],
     ['Created', webhook.createdAt],
     ['Updated', webhook.updatedAt],
diff --git a/test/commands/developer-webhooks.test.ts b/test/commands/developer-webhooks.test.ts
index 35c368ac..b79c1e4c 100644
--- a/test/commands/developer-webhooks.test.ts
+++ b/test/commands/developer-webhooks.test.ts
@@ -23,6 +23,7 @@ const state = vi.hoisted(() => ({
   createSecretAvailable: true,
   listPages: [] as Array<Record<string, unknown>>,
   rotationSecretAvailable: true,
+  savedEventTypes: ['message.received'] as string[],
 }))
 
 vi.mock('@polymorfa/sdk', async (importOriginal) => {
@@ -51,6 +52,7 @@ describe('developer webhook commands', () => {
     state.createSecretAvailable = true
     state.listPages.splice(0)
     state.rotationSecretAvailable = true
+    state.savedEventTypes = ['message.received']
   })
 
   afterEach(async () => {
@@ -177,6 +179,7 @@ describe('developer webhook commands', () => {
     const text = output.mock.calls.map(([chunk]) => String(chunk)).join('')
     expect(text).toContain('URL\thttps://hooks.example.test/polymorfa')
     expect(text).toContain('Event types\tmessage.received')
+    expect(text).toContain('Phone offline warning\toff')
     expect(text).toContain('Status\tsucceeded')
     expect(text).toContain('Last status code\t204')
   })
@@ -366,6 +369,146 @@ describe('developer webhook commands', () => {
     })
   })
 
+  test('sets the phone offline start day on create', async () => {
+    const directory = await mkdtemp(join(tmpdir(), 'polymorfa-command-'))
+    directories.push(directory)
+    const input = join(directory, 'webhook.json')
+    await writeFile(
+      input,
+      JSON.stringify({
+        eventTypes: ['session.phone_offline'],
+        url: 'https://example.test/hook',
+      }),
+    )
+    stdout()
+    await developerEnvironment(() =>
+      WebhookCreate.run([
+        '--owner',
+        'project',
+        '--project',
+        '11111111-1111-4111-8111-111111111111',
+        '--input',
+        input,
+        '--phone-offline-start-day',
+        '8',
+        '--secret-out',
+        join(directory, 'webhook.secret'),
+        '--idempotency-key',
+        'create_1',
+        '--json',
+      ]),
+    )
+    expect(findCall('webhooks.create').args[0]).toEqual({
+      eventTypes: ['session.phone_offline'],
+      phoneOfflineStartDay: 8,
+      url: 'https://example.test/hook',
+    })
+  })
+
+  test('updates only the start day without an input file', async () => {
+    stdout()
+    await developerEnvironment(() =>
+      WebhookUpdate.run([
+        'wh_1',
+        '--owner',
+        'organization',
+        '--phone-offline-start-day',
+        '13',
+        '--idempotency-key',
+        'update_1',
+        '--json',
+      ]),
+    )
+    expect(findCall('webhooks.update').args.slice(0, 2)).toEqual([
+      'wh_1',
+      {phoneOfflineStartDay: 13},
+    ])
+  })
+
+  test('turns warnings off by removing the event from the saved event types', async () => {
+    state.savedEventTypes = ['message.received', 'session.phone_offline']
+    stdout()
+    await developerEnvironment(() =>
+      WebhookUpdate.run([
+        'wh_1',
+        '--owner',
+        'organization',
+        '--phone-offline-start-day',
+        'off',
+        '--idempotency-key',
+        'update_1',
+        '--json',
+      ]),
+    )
+    expect(findCall('webhooks.retrieve').args[0]).toBe('wh_1')
+    expect(findCall('webhooks.update').args.slice(0, 2)).toEqual([
+      'wh_1',
+      {eventTypes: ['message.received']},
+    ])
+  })
+
+  test('refuses off when it would leave no events', async () => {
+    state.savedEventTypes = ['session.phone_offline']
+    const stderr = vi
+      .spyOn(process.stderr, 'write')
+      .mockImplementation(() => true)
+    const error = await developerEnvironment(() =>
+      WebhookUpdate.run([
+        'wh_1',
+        '--owner',
+        'organization',
+        '--phone-offline-start-day',
+        'off',
+      ]),
+    ).catch((caught: unknown) => caught)
+    expect(exitCode(error)).toBe(2)
+    expect(String(stderr.mock.calls.flat().join(''))).toContain(
+      'at least one event',
+    )
+    expect(state.calls.some(({method}) => method === 'webhooks.update')).toBe(
+      false,
+    )
+  })
+
+  test.each(['6', '14', '9.5', 'never'])(
+    'rejects --phone-offline-start-day %s before calling the SDK',
+    async (value) => {
+      const stderr = vi
+        .spyOn(process.stderr, 'write')
+        .mockImplementation(() => true)
+      const error = await developerEnvironment(() =>
+        WebhookUpdate.run([
+          'wh_1',
+          '--owner',
+          'organization',
+          '--phone-offline-start-day',
+          value,
+        ]),
+      ).catch((caught: unknown) => caught)
+      expect(exitCode(error)).not.toBe(0)
+      expect(String(stderr.mock.calls.flat().join(''))).toContain(
+        'Expected a day from 7 to 13, or off.',
+      )
+      expect(
+        state.calls.some(({method}) => method.startsWith('webhooks.')),
+      ).toBe(false)
+    },
+  )
+
+  test('requires an input or a start day for updates', async () => {
+    const stderr = vi
+      .spyOn(process.stderr, 'write')
+      .mockImplementation(() => true)
+    const error = await developerEnvironment(() =>
+      WebhookUpdate.run(['wh_1', '--owner', 'organization']),
+    ).catch((caught: unknown) => caught)
+    expect(exitCode(error)).toBe(2)
+    expect(String(stderr.mock.calls.flat().join(''))).toContain(
+      'Provide --input or --phone-offline-start-day.',
+    )
+    expect(state.calls).toHaveLength(0)
+  })
+
   test('creates a test event and delivery without an extra confirmation', async () => {
     const output = stdout()
 
@@ -660,7 +803,7 @@ function resource(
               response({
                 createdAt: '2026-09-08T00:00:00Z',
                 enabled: true,
-                eventTypes: ['message.received'],
+                eventTypes: state.savedEventTypes,
                 format: 'native',
                 headers: [],
                 id: String(args[0]),
diff --git a/test/developer/render.test.ts b/test/developer/render.test.ts
index f022b578..a5445800 100644
--- a/test/developer/render.test.ts
+++ b/test/developer/render.test.ts
@@ -60,12 +60,34 @@ describe('developer detail renderers', () => {
       'Enabled\ttrue',
       'Format\tnative',
       'Event types\tmessage.received, session.connected',
+      'Phone offline warning\toff',
       'Header names\tx-team',
       'Created\t2026-09-08T00:00:00Z',
       'Updated\t2026-09-08T01:00:00Z',
     ])
   })
 
+  test('renders the phone offline start day for a subscribed webhook', () => {
+    const base = {
+      createdAt: '2026-09-08T00:00:00Z',
+      enabled: true,
+      eventTypes: ['session.phone_offline'],
+      format: 'native',
+      headers: [],
+      id: 'wh_1',
+      owner: 'project',
+      projectId: '11111111-1111-4111-8111-111111111111',
+      updatedAt: '2026-09-08T01:00:00Z',
+      url: 'https://hooks.example.test/polymorfa',
+    }
+    expect(renderWebhookDetail({...base, phoneOfflineStartDay: 8})).toContain(
+      'Phone offline warning\tfrom day 8',
+    )
+    expect(renderWebhookDetail(base)).toContain(
+      'Phone offline warning\tnot reported',
+    )
+  })
+
   test('renders delivery status and last outcome', () => {
     const text = renderDeliveryDetail({
       attemptCount: 2,