This happens to me more frequently than I would like. The bluetooth connects properly as per the message on the LCD, the phone call connects, but I am unable to hear anything from the party I called. When possible I turn off the car and restart. Sometimes, reestablishing a new connection will solve the problem but not always. Its frustrating but have been unable to determine the cause. I am using a Blackerry curve. Not sure what phone you are using but I dont think its a particular phone issue. It appears to be a bluetooth issue.
2006 GS300 - 53K miles. Just happened to me today. Deleted the phone from the touch screen control. Re-linked the phone to car and.. nothing, zip. Using a Sprint Lotus cell phone.