iPhone - calls don't work, text and data OK. Specific to sim and phone

Hi, I'm having a real head-scratcher after trying every troubleshooting option under the sun..

Basically, I've ported my number from another phone provider. I received their sim card and it doesn't make or receive calls (data and text are OK), and the reception is quite sketchy.

But here's the weird part: when I test the new sim in another phone - the sim can make calls fine, so the sim works. Then, when I try another sim in my phone, that sim can make calls. So the phone works OK...

For whatever reason, this specific phone providers sim in this specific phone can't make calls, and has trouble finding reception.

I've called apple, the phone hardware got a green light after a diagnostic. I've reset network settings, reset the entire phone, I've bought a brand new replacement sim, the phone isn't locked - all leaves me with the same issue.

I'm wondering whether something when wrong in the porting process, or something on my phone isn't written correctly as it relates to this specific provider. My provider is looking into it now

Appreciate any help as I'm all out of normal troubleshooting options.