As a phone I can't help you.. ONSTAR serves many other functions as well.. One of them used to come in handy when I was working.
Neighbor PD put out a BOL (Be On Lookout, what the TV dicks really mean when they say "All Points Bulletin") and after several re-reads I realized they had not tried to find the Esclade using ON-Star (All Esclades come with it).. I sent them a phone number (1-800-###-####) and 15 minutes later they teletyped back the address where the car was parked and a request we check for it.. It was there, Engine still running.