Sony-Ericsson W580i makes calls on its own!

Hi,

I've just been given a W580i. I had locked the handset and put it in
my pocket. When I got it out of my pocket I discovered it had called
someone (in my address book) and the call was still in progress. The
handset was closed (it's a "slide" type). Voice control is not
activated.

I checked the handset was still locked. Is this a fault or am I doing
something wrong?

M.