The following features of the phone contribute to the problem:
When the phone has not been used for a while, the screen goes blank and the phone locks. In order to unlock the phone, you press either the 'Power' button or the 'Menu' button to bring up the phone unlock screen.
From the phone unlock screen, you slide the green slider from left to right to unlock the phone (much like the iPhone). However, there is a bug (or mis-feature) on this screen. If you press the 'Menu' button, the phone also unlocks.
When the phone is unlocked, you get to the Home screen.
When you press the 'Call' button from the Home screen, it brings up the call log. When the call log is active, pressing the 'Call' button dials the most recent entry in the call log. This is meant to make it easy to return a missed call, or redial the person you last spoke to.
When the phone is in my pocket, something presses buttons: wallet, keys, buldging thighs, etc.
The 'Call' and 'Menu' buttons are right next to each other. So it is conceivable that they get pressed in a random order.
The sequence is to make unwanted calls is:
1. The phone is in my pocket with the screen blank and the phone locked.
2. Something presses buttons randomly.
3. Pressing 'Menu' brings up the unlock screen.
4. Pressing 'Menu' unlocks the phone -- this is a bug.
5. Pressing 'Call' brings up the call log.
6. Pressing 'Call' redials the most recent entry in the call log.
7. After the called party hangs up in disgust, the phone returns to the call log screen, ready to make more calls from step 6.
8. It takes a while for the phone to time out and lock itself again. Even when it does lock, it can still make unwanted calls from step 2.
One work-around is to keep the call log empty. However, if I miss a call, it gets added to the call log, so it is hard to ensure that the call log is always empty.
I have instead enabled a different phone unlock option. Instead of having an iPhone-like swipe-to-unlock, the alternative unlock screen requires you to draw a pattern. It is a bit more difficult to use, but does not unlock if you press the 'Menu' button.
I prefer the swipe-to-unlock unlocker> Is is possible to install another phone unlock screen?