I have had a few instances where my mobile signal just dropped and my SIM PIN screen appeared, I managed to fix that by flashing an other modem. No more issues since then.
The only remaining issue is the call quality over BT. When making a call from my car the sound quality on the receiving end is so distorted it's making a decent call almost impossible. The sound level on both sides is OK and also the sound quality in the car itself is perfect.
It looks the mic function on the BT side needs to be tweaked, can it be it's gain is set to high to overcompensate for the low sound reported earlier?
Using the 28/9 build with stock kernel and settings.