FP3 connectivity issues

I don’t think that’s the case, since the topic you point to, while post-A11 like this one, is about mobile data and IP issues. Android 11 brought lots of temporary problems of different sorts, but those related purely to mobile data have mostly been fixed through adjustment or reinitialisation of APN settings, or taking the phone down to 2G and back up to 4G. It’s unwise to lump things together too hastily.

The problem we’re dealing with in the present topic involves loss of basic telephony (and not surprisingly, but as I see it only as a consequence, loss of data too).

I’m rather inclined to think that these call-related issues are coming from the combination of FP3 + A11 + specific carriers.
In view of that, it may be up to the carrier, or FP and the carrier, to fix the problem. It’s certainly important to make sure that both FP Support and the carrier are aware of each individual case.