I have the unlock code, how do I unlock my razr V3xx?

It was AT&T, and now I want to use it for T-Mobile. The unlock code I have is a genuine unlock code from AT&T customer service.

I’ve tried putting (1) the original AT&T SIM in, (2) no SIM, and of course (3) my T-Mobile SIM that I want to be able to use. None of these prompts me for the code, and when I do type the code it anyway, the phone won’t recognize it.

When I turn it on with the T-mobile SIM, it just says “Contact Operator”.

With no SIM, it says “Insert SIM”.

And with the AT&T SIM, it says “Unregistered SIM”, but has most of the normal functionality, such as address book, calculator, etc, but no obvious “Unlock Your Phone” option.

Help! How do I unlock it???