SIP Trunk registration failing after Zendesk voice migration

Does anyone understand why the SIP trunk keeps dropping after mapping Zendesk phone numbers to Genesys Cloud? The migration guide says it should be straightforward, but the logs show a 408 Request Timeout on the REGISTER request.

  1. Created the SIP trunk in Genesys Cloud with the carrier credentials.
  2. Mapped the Zendesk phone number to the new trunk.
  3. The trunk status flips to ‘Registered’ then immediately to ‘Failed’ within 10 seconds.