Hi,
I have the same issue.
See my observations and reflections:
Phone orientation / Dialed person position in phone book:
I have tried to watch when the phone is manually locked before entering in my pocket. I also observed that the phone is pointing outwards or inwards to the body when it is in my pocket. Interestingly enough, my phone continued to dial random people.
No matter whether I have heard them recently or long ago and no matter whether they are at the beginning, middle or end of my phone book.
Siri:
Assuming that Siri dialing someone - I have no explanation how this happens when I driving alone in my car and not talk to anyone (and I do not listen to the radio).
Buttons interaction:
Home button is flat. Theoretically it should not be able to press in your pocket or backpack unless something touches it. Only remains to look at relief buttons for volume and on/off. If there is some combination of pressing which causes the dialing I would accept that as an option.
I wonder if I have a hardware problem why it does not occur when the phone is left on the table?
Аfter all above, the only thing that comes to my mind is:
I guess It can be temperature / humidity or just mechanical issue (when my phone is bended in my pocket).