Can’t send messages with T-Mobile, Error 50

I just wanted to confirm that the phone is now working. It was giving me an error 50 because TMO was BLOCKING my account since it was a new phone and I think it was international and had never been used. They had to get do *#06# to get my IMEI and then register me on the network. I was on the phone for about 30 minutes. They had to escalate it to someone with authorization to unblock me since it was about Sunday when I called. Then they said would call me back in an hour since the person wasn't immediately available or it took time to get registered. After about 1 hour the person called me back and we tested the SIM on the old and new. It wasn't initially able to receive messages but could send. After they unblocked a few more things it worked on the Fairphone 4. I also took out the SIM and tried it on the old phone again and it took about 5 minutes and it registered the old phone handset again which is what I wanted so I could swap back and forth. The rep said it takes a while but it does take time to swap between handsets but they had unblocked me entirely. After the call I put it back in the new handset again. So it works in the USA this phone. It does sometimes go to H++ or 4G depending on area, but usually is 5G. I have no major problems with this phone and seems like an upgrade and faster and more stable. The battery is slightly less capacity than my old handset that had an after market battery for a non-Fairphone handset. Also the Android 11 operating system on this handset is nice, but I don't like that Google makes sdcards restricted in the name of security. I like being able to access my disks. All in all this seems to be working now.

The VoLTE suggestion above seemed irrelevant, but thanks for the help, I just broke down and called their help support (wasn’t sure they supported the new phone but they did!)

No APN settings needed to be modified as they were recognized by the phone already. It was a SIM IMEI blocking and registration issue on TMO's end. Apparently they blocked the phone from sending texts but I could get internet still.