Calls are realtime, voice notes you can listen to, ponder and reply later.
Also, there are no interruptions, because each one is, of course, in the form of a monologue.
Don't hate it until you try it. My primary mode of digital communication with some friends is almost 100% like this - but it depends on the person, I don't want to do this with the other 98%
And me, personally, I never take calls. Hell, I don't even have notitications enabled so I would miss them anyway. Calls are intrusive to me, if wifi (Signal only) is on, I'm doing something on the phone, and being suddenly interrupted won't make me happy.
tldr: different things work with different people. If you don't want this person to send voice notes, just tell them so.