What can I do to stop my Nokia Lumia 530 from making calls when the screen is locked

[Windows Central Question]

I have put a pass code on my nokia Lumia 530 to stop is calling my contacts when it is in my pocket. This has been working until today when it made a call that I was unaware of while it was locked. What can I do to stop it from happening as the call wasted $5 of my credit on one phone call