How to stop the phone from unlocking in a pocket when I activate voice control through my headset?

[Windows Central Question]

I've recently bought a new wired headset with a button to answer calls, etc. When I tried to make a call using voice control, I held the button for a couple of seconds and, as it turns out, the phone unlocked while in my pants pocket and registered a press on the home button, thus stopping voice control. I'm wondering if there's any way to disable this so that I can use voice control to its full extent without having to take my phone out of my pants. I also remember using voice control about a year ago without any issues, so maybe this bug was introduced only in WP 8.1.

P.s.: my phone is Nokia Lumia 820 running Lumia Cyan.