WebRTC STUN connectivity failure on BYOC edge in Asia/Singapore

Does anyone know why WebRTC softphones fail to establish media streams against our BYOC edges in the Asia/Singapore region? The signaling completes, but the browser logs show iceconnectionstate: failed with no UDP traffic on port 5349 despite the firewall rules being open.

webRTCConfig:
 stunServers:
 - turn:stun.us-east-1.aws-us-east-1.prod.public.nice-incontact.com:3478
 iceTransportPolicy: all

The trunks are registered correctly via SIP, but the direct media path seems blocked or misconfigured for the softphone endpoint.