Thank you for clarifying - I think I understand now… “4G Calling” is actually VoLTE?
I have disabled it now and will try to figure out why I’m losing connectivity. I tried Fossify Phone, but the same thing happened in about a day, so I reverted to the original Phone app where I can at least set the call forwarding.
To be clear, what happens is: someone is calling me, but I’m not reachable for them. There is no indication of anything wrong on my side, signal looks good. If I try to call, I get the silence for about a minute (no ringtone or any message), then call is just cancelled. Soon after, I receive an SMS (about the missed call) and then everything works correctly again.
I will try to see if the problem could be connected to battery saving settings or sth. similar.