2009 Prius Recently Purchased: Odd Problem with Bluetooth and Treo 755p

After some initial difficulty synching my bluetooth to my "new" used 2009 Prius, I have had no difficulty making calls from the touchscreen. However, I learned recently that incoming calls do not ring; I know the calls are incoming, however, because my radio kicks out as it should, and my phone rings, but the screen stays unchanged, the Prius' speaker does not ring, and the only way I seem to be able to pick up the call is if I have my phone out and hunt around for the "answer" button on the screen. Once I do that, the Bluetooth seems to work fine and I can carry on a hands-free conversation.

I searched for settings that I may need to change; the only thing I found was one that should automatically answer the call after several rings. However, that doesn't seem to solve the problem.

Other than replacing my Treo (I expect to do that in about six months, but not yet), any suggestions? And if anyone else has experienced a similar problem, does upgrading phones help? Thanks in advance.