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.

Edit - I'm hoping that turning off 'turn control' will solve this, as it started when I enabled that.

Is anyone else confused by the fact that each 'recent' contact only appears once - so if I call someone on on Monday, and then again on Thursday the record of the Monday call is lost?