[username190]

AT&T shut down their GSM 2G network in 2017 - the remaining circuit switched calls were going out over UMTS (3G) until the shutdown.

[p_l]

The radio interface used is 3G/4G, but it still used circuit switching instead of IMS (which uses GPRS channel - just like your 3G/4G/5G data - and SIP, iirc)