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.
Interestingly this "update" looks to be an old fix for the Treo 750 found here and brought to Palm's attention here, though a complete tear-down of the official .cab fix needs to be done to confirm.
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.
Get the official patch/update from Palm here. For those on the go, here is the OTA update link. And remember that you'll have to re-apply the fix if you hard-reset your phone.