Changing carriers and sims - how do I know if the new network will work? My family is considering switching to T-Mobile, and I would like to bring my current Droid 4 with me. I have already stopped by a local T-Mobile store to see if their sim would work. The phone displayed the list of available mobile networks, but I could not make a call. Is that because the sim was not registered to my phone, or because the phone could not access the network properly?

Note, I am aware that a stock droid 4 is locked out of US GSM usage - as far as I can tell I overwrote the NV data to allow this.
Good choice (T-Mobile). If your phone is unlocked you should be able to use your device with most carriers, especially the ones who activate service with a SIM card. 

It's most likely because it's not registered yet, I wouldn't see any reason otherwise. 

Update

I'll agree with Eldrerathis here. It's possible that the device cannot connect to the network correctly.