https://us.community.samsung.com/t5/Galaxy-Note-Phones/quot-Sip-quot-in-text-message-contact/td-p/2602027

Same here

How can u tell if a hacker has eavesdropped on your txt or calls? Besides the key verification because not every iPhone displays it in the Apple ID. I have a 15 and my friend has a 16 max pro she doesn’t have the key verification && my sons older iPhone doesn’t either.

The same thing has been happening to me for just 1 of my contacts. It's so aggravating. Their name displays as my contact originally, then as I send the text it changes to displaying the phone number. When the text message fails it changes to sip:number@msg.pc.t-mobile.com;user=phone. If this contact happens to be in a group chat of mine it fails to send my message to the entire group chat. I have to remove this contact from group chats to be able to send a message to the group. It's unfortunate that this is a problem for so many and T-Mobile hasn't fixed it. I've been experiencing the issue for about a year.

Called T-Mobile. They have never heard of it even though this has been going on for months. Still no solution. I did find out it means it is a VOIP issue. I have tried every troubleshooting issue and nothing. If the person texts me sometimes if I catch it in time it will send. The only number I am having an issue with is my son’s. He is on the same phone plan as me. 

Following incase some tech savy person can finally figure this out.

Any solution for this as of 12/31/2023?

Nope, it's my dad's number :)

Google the number having the issue. This is a long shot, but that number may be cloned by another phone/system. I googled the number I'm having a problem with and it shows as a Russian number/owner. This may be a non-point but it seems weird.

I tried all the fixes mentioned in this thread and another thread and nothing is doing the trick. As of December 29th, after a recent Android update, the problem returned after being resolved for a short time.

Come on t-mobile, and get it together!

Following

Rep asked me to delete and add the contact.

Then did the clear cache..

If not install an alternative messaging app. I have message + from verizon.

Is the message app itself 

This didnt work for me it changes the person to “ SIP: phone number @msg.pc.tmobile:user=phone” everytime and its just one number. I can recieve messages from them but they dont get mine when i send them.