Backup handling for V2 Voices

On V1 we built our own API endpoint to fallback to when the Synthflow endpoint did not return a 200 to Twilio.

Because of the new SIP logic this is not possible in V2. A plan on how to allow the call to not drop when Synthflow returns an error would be great. Especially after a day like today where all calls were dropped between 9AM-1PM.