You mean this thing makes calls too?!?! It just keeps getting better and better.
All kidding aside, it's fine. It's good. Same as other iPhones. If on VOLTE or wifi calling and connecting with someone else who's doing either also, it sounds as if they're standing next to you--even when on speaker phone.
My wife who has a 5c and views these things as phones says I might not be happy with it because of call quality. I who have a Lumia 520 and wish to get one of these for obvious reasons was interested in other opinions.
Call quality seems the same as my old iPhone 6. No one yet has said they couldn't hear me either. I'd just get one and try it for yourself..you have 14 days to change your mind.