I have a 6a as well. Just tried it. I can change the volume but it never goes down to muted, even if i turn the volume down to zero on my device.
The volume goes down a little bit but not as much as it should. On every other phone I owned i was able to mute the volume when lowering the volume to zero. Here, the call is still very good hearable when i reduce the volume to zero.
Doesn't bother me though, the volume is OK for me, but yes, it's not the expected functionality.