I live in Europe, but I can share my experience.
I receive 100% of the calls and SMS, no issues whatsoever. MMS I don’t know, because I never managed to understand my carrier documentation properly.
DATA:
In my home country, sometimes the data connection goes stale and requires a full modem hard switch (rarely data restarts after switching data off and on in Settings). Careful observation made me think that the issue happens after exchanging a few SMS.
When data is dead, I still can make and receive calls and SMS.
HOWEVER
I spend a good part of the year abroad in Germany and in Slovakia and in both countries data connection is rock solid.
Once I got in touch with my carrier’s support hotline and, well, they have no idea what a Librem 5 is. When I go to the support portal, my phone is listed as “Unknown device”. When I had an iphone, it was correctly identified, including generation.
The provider said they can’t help troubleshoot an unsupported device and that was the end of it.