[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.

[POCKET_SANDO]

This is how researchers were able to remotely hack several Chrysler models. They used a Sprint hotspot to get an IP on the cell network and were able to connect to any other Sprint device. The cellular modems on these cars were on Sprint so they just had to be on the same network. I wonder if Verizon intentionally blocks this.

https://www.kaspersky.com/blog/blackhat-jeep-cherokee-hack-e...