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.