I have the same issue! I must have bumped my phone against a surface at some point and now the volume button is permanently pressed. Fortunately, my phone is powered on so i can still use it but i cant un press the button. This happened to my one plus one and when it did, i unscrewed the device and used pliers to straighten the "bar" which is the volume controller and then re-assembled the phone. I cant find any video or tutorial on how to unscrew the one plus two however..... :(