Bleeding Magenta thank you so much for sharing this. I forwarded those ports to my phone and so far so good 
I never really solved it. Couldn’t figure out how to open ports on my router for multiple phones per the Tmo recommendations, so sorta gave up. It did seem to start working better after a while, but still not perfect.
Another solution you might want to look into is getting a Tmo microcell. https://www.t-mobile.com/support/coverage/4g-lte-cellspot
I haven’t used one of these with Tmo but I did use one with ATT a while back and it worked great. It wasn’t expensive.
How long has it been working since you did that? I tried that twice. Works for a while and then the problem comes back.
Even just rebooting the phone fixes it for a day or so, but it always comes back.
I’ve been following the Google Buganizer issue tracker, and you’re right, some people report having this this issue on other carriers, but T-Mobile does seem to be the most prevalent. This also appears to be an issue on Pixel 6’s so hopefully it gets some love one of these days. It’s been a year with no updates from Google, but the issue was recently added to some “hotlists” whatever that means.
The Pixel December 2022 update finally fixed this issue! T Mobile is blocking the update for some reason, so you may still be on the November update. However, you can sideload it which isn’t too hard if you have a PC.
Edit: Note there is apparently a Bluetooth issue with the Jan update. So if you sideload, just do the Dec update for now.
I may have had luck by selecting “Enable IPSec Passthrough” buried in my router settings. 
Pixel 5. Also my wife has a Pixel 7 but she never had any issues with it to begin with.
FYI, I just experienced the issue again and I have all the latest updates included including the September patch.
Good luck. This was happening on all our iPhone devices if that helps any. I had to open the three ports individually for all the router assigned addresses for the iPhones as I didn’t want to blanket open the ports for all devices. Hopefully it works for you.
Looks like they still don’t have a cellspot for 5G. People have been asking for it for over a year 
https://community.t-mobile.com/network-coverage-5/5g-cellspot-14294
Since I’ve done this, I haven’t had any issues.
The February update finally went through (first since November for some Pixels) and it may have fixed the issue.
Bummer. I’ve actually not had the issue in the 5 days since I installed the Sept update, which is much longer than usual. Could just be luck. The only other thing I did was change it to prefer wifi calling over cellular when both are available. I doubt that makes a difference but who knows.
Good call, I’ll do that too with my two phones as those open ports had me a little skittish. Weird that I never had this issue with Verizon. Maybe a different implementation.
Even a 4g would work for calls though. For data you’d have your wifi right?
I still have this issue. I don’t use GV and its only when I call someone with an Iphone. I have a Moto Stylus 5G (2021)