Are you sure it's autolock? It should not lock when you're calling or answering phone calls.

I still remember the first time I use it and the screen goes blank, then I keep press the side unlock trigger. Later I found out... it's the proximity sensors... It will turn off the screen when you cover the proximity sensors (e.g when you listen to the phone), this is to save power. When you move the phone away from your ear/head, the screen should comes back in a moment.
You can try make a call, then cover the top part of the phone with your hand (where the proximity sensors is), the screen will go blank, then uncover it and the screen should comes back.

It shouldn't lock, because there're times you need to access the keypad (e.g. when calling voice operated options such as 1800 lines, where you need to press the numbers to proceed).