[Droid Ultra General] br0adband:
Sprint and Verizon use a sort of hybrid technology with respect to their 4G LTE networks, that's why their devices require a SIM card. Verizon devices since the summer of 2013 are pretty much always GSM unlocked even here in the US for domestic operation - they come that way because of some requirements that Verizon was responsible for honoring during the frequency allocation auction by the FCC a few years ago.

Basically that means every Verizon device (that is a 4G LTE one which means pretty much everything they've been selling for 2 years roughly) is ready to go by inserting a SIM card (activated for whatever other carrier you use, of course) and running with it. You may be required to fix or alter or add the proper APN for the other carrier - the info is supposed to be on the SIM card and usually is read without issues but sometimes it's not and has to be manually put in which takes a minute or two, not a big hassle.

I'm suspecting the AT&T SIM card you tried originally didn't have the proper APN data (older SIM cards tend to not be read so well in newer devices) and it just wasn't able to work the data connection - the phone call aspects will pretty much always work and you'll know that by the fact that it connects to the carrier you've chosen but that's just for phone calls, not the data.

Sprint locks their devices not only with a SIM lock but also a domestic lock of sorts: they don't like people using the devices they sell (contract or not) here in the US so they will block the use of SIM cards for providers here in the US even if you get the SIM unlock code from them more often than not. This isn't a 100% sure thing but it does tend to be reported quite often.

Workarounds for the Sprint issues usually relate to finding something like a radio file/firmware from a Sprint device used someplace else in the world and then flashing it on a domestic device for use here in the US, or another method is patching some system files to allow the radio to work properly without such artificial limitations put in place by Sprint's somewhat draconian methods.

Anyway, with a Sprint device here in the US, the LTE bands that Sprint does use are completely different from those that AT&T and T-Mobile tend to use: Sprint likes bands 25, 26, and 41 while AT&T likes 2, 4, and 17 while T-Mobile favors 2, 4, and 12. Verizon likes 2, 4, and 13 - see how this works out?

There's overlapping coverage from AT&T, T-Mobile, and Verizon on the 2 and 4 bands hence devices sold by these companies being able to mingle, so to speak, on the GSM operations - Verizon phones of course still require CDMA for their primary usage so if you get a Verizon device with a busted SIM card slot or whatever it won't be of any use on AT&T or T-Mobile.

But that's it, more or less. The Droid series of devices since the Droid Ultra (MAXX and Mini came later) have been GSM unlocked from the go, including the Droid TURBO as well and most everything else Verizon sells. I've heard a few reports that newer devices (like the Droid TURBO perhaps) may have another level of blocking of some kind but, so far every device I've owned from Verizon since the Droid Ultra was released has been GSM unlocked and works fine with my T-Mobile service with nothing more than manually adding some APN info in 1 min or less.

Hope this helps...

Of course it's possible, you should check out AT&T coverage maps and cross-reference the locations to get more info, or just call AT&T and ask them for specifics, it can't hurt.