Re: Power button ends calls on 6s
I'm having the same problem on my iPhone 6 on iOS 9.3 beta.
I think it's specific to bluetooth devices. Normal behaviour without a BT peripheral is that the lock button ends calls when not on speaker, but doesn't end them when you are on speaker.
Usually, when a BT headset is connected, the speaker button on screen shows a BT logo and gives you a list of the various audio options, including your BT headset (default), iPhone and iPhone speaker. I've just noticed that this is not the case on 9.3 - even with a BT headset connected (and working), the button is still just a toggle for speaker off and on. Even the airplay menu in control centre doesn't show the BT headset, it only appears in the list of BT devices in settings.
The workaround I have found is to start your call, turn on the iPhone speaker, lock the screen then press the call button on my bluetooth headset, which automatically routes the call to the headset but keeps the screen locked. I can't be sure this will work with all headsets and is pretty annoying.