Recently, I've been disappointed to find my Treo 650 turning off the phone radio by itself. If I was lucky, I heard the melodic sound that means "radio now off", and then I manually turned the radio back on. If I was unlucky, I didn't hear it, and missed phone calls and messages.
It seems that the cause is that the Treo 650 SIM case gets looser with wear, and any break in connection between the SIM and the phone causes the phone radio to be deactivated. The solution is pretty easy, and described at the end of a FAQ I came across. In summary, take out the SIM tray from the phone, remove the SIM. Put several layers of paper (cut small to fit) in the SIM tray, and then clip the SIM back on top. When you put the SIM tray back into your phone, it should work reliably again, with no more random turn-offs!