In the last few days, Ive had 4 phone calls come through the system and all are wrong numbers!!! Makes me wonder if someone from Onstar is calling at random. Each call takes a minute off the "minutes remaining"
I've heard of this happening quite a few times. Heck, even Edmunds.com has had it happen on their long-term Buick Enclave, and found it funny at least.
Apparently I think the issue is old Verizon/Onstar numbers no longer in use, but that once were quite a while back, now re-assigned (just like what can happen with a cell phone in general) and sometimes, by a 1 in 10 (or, whatever it is) chance, you'll get one that a person or company still has in their list.
If it was Onstar calling, you'd know it was Onstar. Otherwise, it sounds like the above. I would actually do an Onstar call while in the car (blue button) and explain to them what has been happening, and see if they can do anything, like maybe swap your number or have more details on the calls.