I wonder if it got linked that account when you made a phone call on it using the SIM in it that wasn't your SIM. (I'm also curious why you did that, but that's different question.)
What happens when you take that SIM card out and put yours in?
It works, it lets me make phone calls the phone is unlocked. But the phone is linked to another persons account you know? That person can do anything with it. And as another person said, the phone could be on Verizon EDGE. Which is a huge no-no as Verizon can de activate the phone