If my code tries to make a call to a SIP endpoint, and that endpoint is no longer registered, the call fails with error 32009 - Dialing SIP Endpoint failure - User not registered.
From what I've read, the only way to prevent this from happening is from manually searching for the endpoint via https://www.twilio.com/console/voice/sip/endpoints/SDXXXXXXX.
Is there any way to verify a SIP Endpoint is registered with the Twilio API prior to initiating the call?
There is not. Use the DialCallStatus to take an alternate action if the device is not registered.