Some of the earliest LTE devices did data over LTE and voice over 3G. Notably this applies to the iPhone 5s does not support voice over LTE - which you commonly see on Android devices as VoLTE. There’s a variety of other devices in the the same situation. The user experience on these devices often does not make it clear that audio is going over 3G.
This has caused many people to get notified of incompatible devices that they thought would work. The devices do continue to work, for now, but as 3G is shut down in a staggered fashion, they’ll seem like they work until the owner tries to make a phone call or realizes they haven’t received a call in a while.
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)
This has caused many people to get notified of incompatible devices that they thought would work. The devices do continue to work, for now, but as 3G is shut down in a staggered fashion, they’ll seem like they work until the owner tries to make a phone call or realizes they haven’t received a call in a while.