Wh is my unlocked GSM Droid Razr showing No Service with AT&T SIM Card?

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AC Question

Unlocked GSM Droid Razr Shows "No Service" With AT&T SIM Card

I have a Droid Razr that I got from Verizon a few years ago. It's unlocked and GSM compatible; I've used it with Bell Mobility and it worked.

Now, I've cancelled my Verizon service and ported my number to AT&T but my new SIM card from AT&T won't work— when I insert it, the phone declares: "No service" and promptly denies the existence of any cell tower in range. It does, however, read my phone number off the SIM card itself; it just won't connect to any networks.

Note that when I was in Canada, I had a similar problem— when I inserted my SIM card from Bell Mobility, it worked fine but when I inserted the Verizon SIM card it again declared "No service" and refused to connect to Telus (which Verizon uses for roaming). I'm not sure how I fixed it then— I think I just had the phone turned off for a few days and when I got back to the US it found Verizon's network. There was a long conversation with Verizon, though.

Unfortunately, it's a bit harder this time around. AT&T can't do anything besides verify that my SIM card works (in another phone, it worked fine) and Verizon won't talk to me because I already cancelled my service.

When I put the Verizon SIM card back in, it works fine and happily connects to Verizon's network; it even let me call 611 for tech support, which was profoundly unhelpful. With the AT&T SIM card, though, there's nothing.

PS— I posted this in the forum but then I noticed an ask question button, sorry if one or both of these posts is in the wrong place.