The volume of the call increases by itself when I'm using headphones. It starts at max when a call is coming and a minute or so after I lower it it increases again to the max. I thought it was my headphones, but it happens with different ones. The volume button also looks fine. And besides, this increase doesn't happen immediately after I lower the volume (which I would expect if something was constantly pressing a button) I tried to look for a setting that does that, but couldn't find any. I have Galaxy S5 with stock 5.0 ROM
First try to disable the equalizer of your music player. The Call Ring, Music volume and Alarm has different sound settings. So, check if all the settings are set to minimum volume. And if that doesn't solve your problem, install the Volume Limit & Lock app from google play-store.