alankeny , I'm glad you're having this problem.... Not because I'm heartless about your troubles, but because finally I see someone who appears to be having the same problem I've been having.
My situation is almost exactly the same: Latest version of GOS but on a Pixel 6, have a JMP.chat phone number, using the latest version of the Cheogram app from F-Droid. Also, I have JMP's physical sim card for their data only cellular service.
Here's what I've seen:
When relying on Wifi, all incoming phone calls always ring as expected, no problem.
When relying on the cellular network, about 70% of the time all incoming phone calls always ring as expected, no problem. However, about 30% of the time - I can't determine a pattern - if someone calls me, the phone will ring about one to three minutes after the caller gives up and hangs up his phone. Of course, by then when my phone rings and I try to answer it, there is no connection to the caller since he'd long ago hung up.
I post this here only in case this might help to diagnose your problem, I don't mean to hijack your thread, but I strongly wonder if we're dealing with the same underlying problem.