For those in the U.S., it appears that the phone works on either AT&T or T-Mobile towers:
T-Mobile: I tested it out on the T-Mobile MVNO MintSIM. Yes, 3G is not as speedy as 4G/LTE, but it's ok.
AT&T:
An Amazon review reported that it works on AT&T towers using MVNO Cricket Wireless:
"Americans beware that you can only use 3G on our networks, but it hasn't seemed to be a problem. Benchmark: I am able to listen to youtube videos in traffic with only a few buffering hiccups."