iPhone 6 call volume & music problem

When I'm listening to music and a call comes in, sometimes the call volume is very low/quiet. So I try turning up the volume using the side buttons, or headphone buttons -- however they don't change the call volume at all. Then when the call ends, the music comes back on and it's BLASTING in my ears. Is anyone else having this problem?

I'm running an iPhone 6 on 8.1.2.