This is a really annoying issue with this phone. I have experienced it on a few occasions (think 4 times during 6 months at irregular intervals).

The phone somehow drops of the network, such that I can not receive calls or sms. The bad part is that it still indicates that it has a signal. I only find out about this after I try to call someone and cant establish the call.

http://talk.maemo.org/showthread.php?t=49350

this thread discusses your or a similar problem. My phone works again after reboot. I think the issue is that I dont know when it happens. That said it hasn't happened to me after pr 1,2