Comment by Denatonium

In the case of T-Mobile, unsolicited inbound IPv6 connections are blocked, but direct P2P is still possible. I successfully established a WireGuard tunnel over IPv6 between 2 phones. With IPv6, since the internal addresses and ports and the same end-to-end, all that is needed is a dynamic DNS service; STUN isn't necessary. I did need to set a persistent keepalive of 25 seconds on both sides of the tunnel to keep the firewall holes open.

Interestingly, Verizon Wireless blocks connections to other Verizon Wireless IPv6 addresses. T-Mobile-to-T-Mobile connections work, Verizon-to-T-Mobile connections work, but Verizon-to-Verizon connections do not work. Given the way Verizon's network has stagnated while T-Mobile's network has been rapidly improving, it may be time to move away from Verizon.

Slightly off-topic, but if you have a modern Google Pixel phone, Google includes "free" VPN service (which probably collects/sells your data). This service uses Endpoint-Independent filtering, so if you send an outbound packet with the source port you want to map, regardless of the destination IP/port, you can effectively receive unsolicited inbound connections from any host on the internet that contacts your IP:port, so long as you send a periodic keepalive packet from the source port you are using to anywhere.