Following on from Terry, it sounds like it could be something simple like a short on the phone mute lead (and/or other leads) on the radio? during installation of phone kits and the likes, it can happen that the mute lead (or others) become damaged when the radio is seated back in to the chassis. This could cause the damaged wire/s to be earthed off a metal part somewhere behind the radio which causes the mute line to be "energised", causing the radio to believe that the phone is in use which invokes the "phone" function on the radio.
To check for the above, visually inspect all wires going in to the back of the radio, particularly the phone mute lead, and repair any damaged wires with insulating tape. Be carefull when you re-seat the radio back in to the dash.
TEflOn, June 2006