I too have been mystified as to why sometimes it's impossible to get sound through the 2005 Prius speakers with the 700p when dialing out. (It has consistantly worked for incoming calls.) And yes, I know that Palm still says the two are incompatible as of yet. But I had to get clear on why it works sometimes, and why it doesn't at other times. This is what I've discovered so far:

THINGS THAT POTENTIALLY RUIN THE CONNECTION:
1. dialing out from the 700p when there is an existing bluetooth link to the Prius. (always)
2. turning on the Treo bluetooth before turning on the phone. (sometimes)

THE FIX THAT HAS WORKED FOR ME EVERY TIME (so far ):
Taking my portable phone out to the car and calling my Treo 700p cell phone while it is linked to the Prius.

I hope that some of you will try this fix to see if it works for you. Please let me know if it does or doesn't.