I think I've worked out how it is choosing who to phone. They're all people in my 'recently called list', which shows up when you activate the phone mode. I find that every so often the phone will simply activate in its pouch (I wear it in a pouch round my neck in portrait mode some of the time), so I am gently 'bumping' one of the recent contacts, and bringing up the details. In the unlikely event I 'bump' it again, it will call.