I even have this problem. Maybe there is a problem with reconnect to the network when the phone loses connection. At home I have a verry bad connection, so sometimes the mobile network is lost. And the deactivation only happens here. Since today.
I hadnāt had a big problem related to that in forever but this weekend I got a call, answered then the communication didnāt work and the phone didnāt have a network and I cut it (or maybe it cut itself down), I had a bit of trouble getting it up again (but still was quick) and I could finish my communication.