Calls do not use OMEMO, but instead use DTLS-SRTP - which is still end-to-end encrypted as only both devices have the keys for the calls.

That said, as of a few weeks ago, Conversations does use verified OMEMO keys in the handshake to display a shield on the call: https://github.com/iNPUTmice/Conversations/releases/tag/2.9.8 (note that this requires that you have physically scanned the QR code of your contact’s OMEMO key).