No phone is perfect, and the Sprint Treo Pro has some hiccups as well. One that has been documented by many is an issue with the ringer where essentially the settings get corrupted in the registry, resulting in "silent" incoming calls:
The root cause of the Treo Pro No Ringer issue is related to a rare corruption of the Treo Pro’s registry key. This can lead to the smart phone not ringing and causing the user to miss an incoming call.
If you see a Software Version that is T850EWW-1.04-SPT you should install this update.
This also does not address the other missed-call problem associated with fringe reception (documented and addressed here), that is the one where you get no notification at all about a missed call as opposed to just a silent ringer.