Interesting. I thought before it relied either on the T-Mobile or the AT&T networks, but that webpage only lists AT&T (at least currently). Of the two networks, it was my understanding that T-Mobile was the one that cared less about what device you used on their network.
I did more reading on old posts, and T-Mobile definitely had been an option for a network backbone. Apparently Purism gave up on using T-Mobile as a backbone network and is only focusing on using AT&T. That is confusing, however, as it seems that AT&T has shut down its 2G and 3G, leaving only 4G as an option for the Librem 5. Calling on 4G requires VoLTE, so…I can only assume Purism has worked with their AweSIM subscribers to enable VoLTE on the L5 modem and it’s working on the AT&T network? It would be really cool if someone that is not employed by Purism could confirm this.