why does the samsung display locks itself when a phone call is made?

Every time I make a phone call, the display on the screen locs itself, and I have to double tap it, to make it work, as I use pay-as-you-go (prepaid) system, I am unable to type in the code to top-up. How can I DISABLE the lock...?

via Forums de téléphone cellulaire (en français) http://cellphoneforums.net/fr/samsung/t363597-why-does-the-samsung-display-locks-itself-when-a-phone-call-is-made.html