Unlocking a device for use on another USA carrier is not going to happen. The cell carriers did agree to do it in 2015 but the hoops and qualyfers to do it are just to restrictive.

Even if you could do it functionality on the other networks may be little to no use.

AT&T and T-Mobile use GSM. Sprint uses a different version of 3G CDMA not like Verizon's CDMA technology.

The number of devices unlocked for use outside the USA are just for that. Outside in another country not for use here.

And AT&T depending where you live doesn't have lines out the door as you mentioned. Verizon still has the top spot of number one in the industry. Not bad for a company that came into the cellular market after AT&T (Cellular One, Cingular)

Good Luck