SIP Trunk registration fails 408 during Zendesk voice channel migration

Just noticed that the Genesys Cloud SIP trunk fails to register with our Zendesk Voice infrastructure, returning a 408 Request Timeout immediately after the INVITE.

The configuration looks correct based on the Zendesk SIP requirements:

sip_trunk:
 host: zendesk-voice.zopim.com
 port: 5060
 transport: UDP
 auth:
 username: "zdesk_trunk_user"
 password: "***"

Is the timeout related to UDP vs TCP transport differences between Zendesk and Genesys Cloud? We are using the default Architect flow for inbound routing.