IIRC - It's less voice over 3G and more voice over 2G but done on whatever radio interface you have. A question of using pre-IMS voice call interface vs. using IMS one (which was actually introduced with 3G, but first use for plain voice that most people have seen was WiFi Calling)
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)
