Handsets have been known to get issues relating to their software and even though this might take a while before it starts to show up, sometimes, it could even be the fault of the manufacturer themselves when they roll out a faulty update to their users of their units.
While there hasn’t been a root cause confirmed for this occurrence, some customers of AT&T in the United States who also happen to be using the Blackberry Priv have complained that their device goes into “NO service” mode more frequent than usual and in the long run, that affects the quality of calls and anything internet related on the device.
However, Blackberry has identified that they are now aware of the problem and are working in conjunction with AT&T to make sure that the issue gets resolved as fast as possible. This should be taken in good faith and even though a specific timeline was not given, it is normal for BB to roll this one out fast due to the sensitivity of the issue.
For now, the only workaround the problem is to restart your device from scratch when this happens.