For a 2004-2005 Prius, this is listed in the Navigation Owner's
Manual. I'm guessing that the Bluetooth section was moved into the
main Owner's Manual for 2006/2007, but I do not have a copy of one of
these manuals to point out the section. In the 2004 Prius Navigation
Owner's Manual, it's in the "Other Functions - Hands-Free system"
section," specifically on pages 110-112. There's a section for "Enter
the Bluetooth Phone" and "Select the Bluetooth Phone."
I may have mis-spoke. You can have up to 4 Bluetooth phones
registered with the car, but only one at a time may be paired. You
select which phone (of those that are registered with the car) to pair
at any given time.
INFO > Telephone > Settings
scroll down to...
Bluetooth: use the "Entry" button for registering/pairing a new phone
(enter the PASS-KEY)
Bluetooth: use the "Settings" button to verify which phone is
connected via the "Bluetooth Information" screen
Select Telephone: use the "Select" button to choose which of the
entered phones to connect via the "Select Telephone" screen