exactly the same problem here, while making a call would hear ringing (sometimes out of the loud speaker) but when the other end picks up the call it's mute on both ends.
If I or they try to call with whatsapp it's all good.
Only a hard reset will fix it and only for some time, then the same issue represents itself.
I have to say it's very annoying, when you use your iphone for work.
This iOS 11 update is embarrassing. It's totally out of control.
Everybody has a diverse set of bugs or issues, which I have never seen before.
Appalling