What OS will actually work with phone calls?

[TRS-80]

I have been using Mobian with a prepaid SIM from Walmart (their own Family Mobile service, with GSM (T-Mobile I think)) and so far pretty good. In fairness I only used it extensively for a few days so far. I do drive a lot for work (all over U.S.) so looking forward to doing lots more extended testing once I go back (I am taking at least this whole week off).

There are some glitches, some times the modem drops out and I need to reboot, and it takes some times a few moments to wake up and reconnect to to the network when the phone wakes up (as have been widely reported, and currently under active development as I understand) but if you are willing to deal with these minor inconveniences, it can actually already be used as a phone, including VoLTE which was a very pleasant surprise as I think that is becoming just about required moving forward.

As an added bonus, I was able to sign up for service on their website using a completely fake name (although you will need to provide some email address, anonymous and temporary services are available). No ID nor any other intrusive data collection required. I was quite pleasantly surprised by this.