31001: Application not found.

Twilio returns this error when a call flow tries to use a TwiML App resource that Twilio can't find. In the Voice SDK error list, error 31001 appears as TwiML application not found. In the Error and Warning Dictionary, the same code appears as Application not found (Voice).

• The Voice SDK Access Token doesn't include a valid TwiML App SID in the grants.voice.outgoing.application_sid property, which Twilio needs to route outbound SDK calls through a TwiML App.
• The required TwiML App resource was never created in the account, so there is no Application SID for Twilio to reference.
• A workflow dials a TwiML App via <Dial><Application> but references an app that does not exist in the target account.

• Verify and regenerate the Voice SDK Access Token so that the grants.voice.outgoing.application_sid property contains the correct TwiML App SID for your application.
• Create a TwiML App and use its Application SID in your token or configuration, ensuring the Voice URL is set to return valid Voice TwiML. You can create and manage TwiML Apps via the API.
• If you are using <Dial><Application> to connect to another Twilio account, confirm the referenced TwiML App exists in that receiving account and is configured to accept calls from other accounts.
• Check Twilio's Debugger error logs to confirm the exact error context and retry after correcting your configuration.

• Voice SDKs
• REST API: Applications
• Getting Started with the Voice Android SDK
• Debugging Your Twilio Application