SIP Trunk 408 Request Timeout during Zendesk Voice Migration

Struggling to configure the SIP trunk for our Zendesk Talk migration. The Genesys Cloud edge is returning 408 Request Timeout when testing connectivity from the Zendesk side.

trunk:
 sip_port: 5060
 transport: UDP
 codec_priority: [G.711u, G.711a]
 auth_method: none

Why is the handshake failing if the firewall rules are open?