Without any notification my mobile randomly stops being able to make and receive calls (and text messages), and there is no indication that there is any issue. WhatsApp is fine and other wifi/data apps are ok it just seems that making and receiving calls can stop working. This is even with Wifi calling switched on. When I find out about the problem (i.e. someone contacts me via another method saying they tried to call me) I have to switch on airplane mode and then switch it back off, which seems to reconnect standard calls and messaging.
There is no way to know that this is a problem so I could be missing calls etc and not know.