diff --git a/lib/signalwire/rest/client.rb b/lib/signalwire/rest/client.rb
index ca73747..831c805 100644
--- a/lib/signalwire/rest/client.rb
+++ b/lib/signalwire/rest/client.rb
@@ -4,6 +4,14 @@ module Signalwire::REST
   class Client < Twilio::REST::Client
     # Per-instance REST API hostname. When set, this client's API domain talks to
     # this host instead of the process-wide configuration or environment.
+    #
+    # The caller must leave +edge+ and +region+ unset on a client that uses this.
+    # Twilio::REST::Client#build_uri rewrites the request host whenever either is
+    # set, keeping only the first label and the last two: an override of
+    # "sim.example.com" with edge "frankfurt" and region "us1" goes out as
+    # "sim.frankfurt.us1.example.com", and "sim.internal.example.com" loses
+    # "internal" altogether. Both failures surface as name resolution errors
+    # rather than as anything pointing back at the override.
     attr_reader :api_host
 
     def initialize(username = nil, password = nil, account_sid = nil, region = nil, http_client = Twilio::HTTP::Client.new, **args)
diff --git a/spec/signalwire/rest/client_spec.rb b/spec/signalwire/rest/client_spec.rb
index 049bbdb..4ab5f30 100644
--- a/spec/signalwire/rest/client_spec.rb
+++ b/spec/signalwire/rest/client_spec.rb
@@ -94,6 +94,42 @@ def stub_signalwire_env(host: nil, api_hostname: nil)
 
         expect(client.api.hostname).to eq('simulator.example.com')
       end
+
+      # Twilio::REST::Client#build_uri splices edge and region into the request
+      # host whenever either is set, so the override only reaches the wire intact
+      # on a client that leaves both alone. These pin that boundary: the caller
+      # has to know about it, because every failure here looks like bad DNS.
+      describe 'interaction with edge and region' do
+        def requested_host(api_host:, edge: nil, region: nil)
+          client = Signalwire::REST::Client.new 'AC123', 'TOKEN',
+                                                service_provider: 'twilio',
+                                                api_host: api_host
+          client.edge = edge
+          client.region = region
+
+          stub_request(:any, //).to_return(status: 200, body: '{}',
+                                           headers: { 'Content-Type' => 'application/json' })
+          client.api.accounts('AC123').fetch
+
+          URI(WebMock::RequestRegistry.instance.requested_signatures.hash.keys.last.uri.to_s).host
+        end
+
+        it 'reaches the overridden host untouched when neither is set' do
+          expect(requested_host(api_host: 'simulator.example.com')).to eq('simulator.example.com')
+        end
+
+        it 'has edge and region spliced into the overridden host' do
+          expect(requested_host(api_host: 'simulator.example.com', edge: 'frankfurt', region: 'us1'))
+            .to eq('simulator.frankfurt.us1.example.com')
+        end
+
+        it 'drops the middle labels of a longer overridden host' do
+          expect(requested_host(api_host: 'sim.internal.example.com', edge: 'frankfurt', region: 'us1'))
+            .to eq('sim.frankfurt.us1.example.com')
+          expect(requested_host(api_host: 'dc.sim.internal.example.com', edge: 'frankfurt', region: 'us1'))
+            .to eq('dc.frankfurt.us1.example.com')
+        end
+      end
     end
 
     describe 'without api_host:' do