After more than 2 months I can say that my workaround for this problem (see 2 posts above) works very well.
Zero missed calls during this period.
Since I don't see any comment related to my previous post, I suspect that I wrote it on a wrong place :/
@GrapheneOS , @other8026 , can we do something that more people be informed about the solution of this annoying problem?
I think that I have found a workaround!
I've installed some app which automatically sends an SMS every 1 hour. That's all.
With that activated for almost 1 month now, I did not receiv any "you were unreachable" notification from my mobile provider. Before that It was almost every day and it was escalated in December 2024 with a few missed calls a day.
Device: Pixel 7
Country - Serbia
Carrier - mts
SIM or eSIM - Physical SIM
Is Wi-Fi calling on or off: I don't have that in settings at all. (Why?)
WiFi - always ON
VPN provider (or "none") - none
Mobile data - always ON
Preferred network type - 5G (enabling/disabling 2G doesn't affect the solution)
Battery saver on or off - OFF
"Do not disturb" on or off - OFF
Phone app - DEFAULT
My assumption was that something (GSM modem or some Android's subsystem) goes to deep sleep or something like that and it's not responsive... That was my idea because I have a missed calls in situations when I don't touch my phone for a few hours. So decided to wake it up periodically.
Please report does this helped on your devices.
AND PLEASE: Can you recommend me a FOSS app which can do the same thing. I'm currently using some app from Play Store and I'd like to replace it with some app from F-Droid.