iOS 15.4 - iMessage won't active. Error message - "An error occurred during activation."

My solution is the one that has worked and I haven’t had any issues since.

I removed the cellular plan from T-Mobile from the phone and I then contacted T-Mobile and had them reassign my Eid to my plan and that solved the problems. Restarted the phone waited for the I haven’t had any issues ever since.

here are the steps that I posted a few weeks ago.

I was trying to troubleshoot this issue (spend a night without cell phone service.)

But i was able to get it working again.

1. Have another phone handy cause you will need it to contact you provider (t-mobile in my case)
2. Make sure that you have access to wifi cause you will need it to gain service back.
3. I removed the cellular plan from my account. so that my phone lost service completely. and no service was associated to my phone.
4. Called tmobile from the other phone and asked them re input the EID in their system on their end.
5. waited a few moments and went to add the cellular plan back to my account and then everything starting filling back in.
6. went to reactivate imessage and facetime and it worked with no issue.

Seems like that fixed the problem.

I'm not sure if the people having this problem are using esim or not but this seems to have done the trick and i am able to get and receive imessage and facetime calls again.