There was a failure attempting to Dial the specified SIP Endpoint. The User specified is not registered.
You tried to Dial a SIP Endpoint that is not currently registered with the corresponding SIP Domain. Please check your TwiML or REST API.
If you are using TwiML Please check that there isn't a typo in <Dial><Sip>username@yoursipdomain.sip.us1.twilio.com</Sip></Dial> and compare against your SIP Endpoint configuration. Make sure the username matches a username in the Credential List used to authenticate the SIP Endpoint with the SIP Domain.
If you are using the REST API Please check the To field and verify there isn't a typo. Make sure the username matches a username in the Credential List used to authenticate the SIP Endpoint with the SIP Domain.