Added troubleshooting page for inbound and outbound call failures

fern/advanced/sip/troubleshoot-sip-response-codes.mdx

@@ -25,7 +25,7 @@ A response can originate from Vapi, your SIP provider, your PBX or Session Borde
   <Step title="Collect the call context">
     Record the call ID, timestamp, call direction, SIP provider, and `endedReason`. The direction determines which signaling path to inspect first.
 
-    You can also retrieve `endedReason` from the [Call object](/api-reference/calls/get-call).
+    You can also retrieve `endedReason` from the [Call object](/api-reference/calls/get).
 
     - For an outbound call, start with Vapi and your outbound SIP provider.
     - For an inbound call, start with the originating provider or PBX and the Vapi phone number or trunk configuration.

fern/calls/call-ended-reason.mdx

@@ -17,8 +17,8 @@ Start here if a call failed and you want to quickly understand what happened:
 
 | The caller experienced... | Look for these errors | Likely cause |
 |---|---|---|
-| Phone never rang | `call.start.error-*`, `assistant-not-found`, `*-transport-never-connected` | Account/billing issue, bad configuration, or Vapi infrastructure error |
-| Phone rang but no answer | `customer-did-not-answer`, `customer-busy`, SIP 408/480 | Normal behavior — callee was unavailable |
+| [Phone never rang](/calls/troubleshoot-call-connection-errors) | `call.start.error-*`, `assistant-not-found`, `*-transport-never-connected` | Account/billing issue, bad configuration, or Vapi infrastructure error |
+| [Phone rang but no answer](/calls/troubleshoot-call-connection-errors#the-customer-didnt-answer) | `customer-did-not-answer`, `customer-busy`, SIP 408/480 | Normal behavior — callee was unavailable |
 | Call dropped mid-conversation | `*-worker-died`, `phone-call-provider-closed-websocket`, `worker-shutdown` | Network issue or Vapi infrastructure error (usually transient) |
 | Assistant went silent or unresponsive | `*-llm-failed`, `*-voice-failed`, `*-transcriber-failed`, `*-429-*`, `*-500-*` | Provider error or credential/quota issue — configure fallback for the affected service, then inspect call logs and test the backup provider |
 | Call worked normally, then ended | `assistant-ended-call`, `customer-ended-call`, `silence-timed-out`, `exceeded-max-duration` | Expected behavior — adjust timeout settings if calls end too early |
@@ -59,7 +59,7 @@ These occur before the call connects, during resource setup.
 - `call.start.error-get-phone-number` — Error retrieving the phone number. Verify the number is imported and active.
 - `call.start.error-get-customer` — Error retrieving customer information.
 - `call.start.error-get-resources-validation` — The assistant, tools, or other resources failed validation.
-- `call.start.error-get-transport` — Error setting up the call transport (Twilio, Vonage, etc.).
+- `call.start.error-get-transport` — Error setting up the call transport. See [Troubleshoot call connection errors](/calls/troubleshoot-call-connection-errors#the-provider-refused-to-place-the-call).
 - `call.start.error-vapifault-database-error` — Internal database error during call setup. Retry or contact [support](/support).
 - `call.start.error-vapifault-worker-not-available` — The call's queue time expired before a call worker became available, so the call was not placed. Retry the call.
 
@@ -178,7 +178,7 @@ For step-by-step transfer debugging, see [Troubleshoot call transfers](/phone-ca
 ### Twilio
 
 - `twilio-failed-to-connect-call` — Twilio failed to establish the call.
-- `twilio-reported-customer-misdialed` — Twilio reported the customer dialed an invalid number.
+- `twilio-reported-customer-misdialed` — Twilio reported the inbound call as busy or unanswered.
 - `call.in-progress.twilio-completed-call` — Twilio reported the call as completed on their side.
 
 ### Vonage
@@ -197,6 +197,8 @@ For step-by-step transfer debugging, see [Troubleshoot call transfers](/phone-ca
 - `call.in-progress.error-providerfault-outbound-sip-408-request-timeout` — SIP 408: request timed out.
 - `call.in-progress.error-providerfault-outbound-sip-480-temporarily-unavailable` — SIP 480: destination temporarily unavailable.
 - `call.in-progress.error-providerfault-outbound-sip-503-service-unavailable` — SIP 503: service unavailable.
+- `sip-outbound-unallocated-number` — The number called is unallocated or out of service. Check the customer number.
+- `sip-outbound-carrier-released-call` — The carrier ended the outbound call before it connected. This is usually temporary, so retry the call.
 - `call.ringing.error-sip-inbound-call-failed-to-connect` — SIP inbound call failed during ringing.
 - `call.ringing.sip-inbound-caller-hungup-before-call-connect` — SIP caller hung up before the call connected.
 - `call.in-progress.sip-completed-call` — SIP provider reported the call as completed.