I would check out ebay for the missing phone that you will need. I think that they make a gsm version for use with Cingular, but the only thing is that you will need another line of service with a new number or you will have to change sim cards back and forth, i really don't think you will want to use the phone for the car because it is old now.

The other option is to add a bluetooth system to your car and if you have a bluetooth phone, everything will be wireless.