Hello,
I ran a PCAP on a T43U provisioned with STUN on unique RTP ports.
I see a one way PCAP of RTP streams from phone to server, but none from server to phone in the PCAP.
I checked that the server on GCP has the firewall rules for UDP 5060-5061 and these ports:
tcp:22, 443, 5001, 5015...