Two potential situations exist for what you describe that I can think of, but the first (below) is not specifically excluded in your post - so please forgive me if it's not applicable:
FIRST (I've had experience of)
One phone receives a random set of jolts that equate to calling a number.
e.g. phone in pocket and it makes a call by itself.
1a) set phone to auto lock after a timeout.
1b) use key lock to lock key presses from functioning
2) back this up further, by creating a false contact e.g. "000Aaa" that will appear at the top of the address book, just in case the first entry of the address book gets called by accident.
SECOND (I've not had experience of)
The phone lying on a flat surface makes a phone call with no external input.
If it's this, please confirm.
My guess would be, it is potentially caused by settings for applications having access to phone functions, i.e. when installed the application settings allow them access to the phone.
Modify application settings to deny applications any phone calling access / access to address book / etc.