Mystery calls by my iPhone

In the last couple of days my iPhone made a couple of ghost calls. I.e. it called some people from my address book, without me doing the actual calls. I think I found the reason for that now. When you hit the home button for several seconds, an iPhone 3GS with iOS 3.x or 4.x will go into voice command mode. This even works when the phone is keylocked. The voice function is not so robust and will misinterpret quite a few background sounds, noises and voices for commands. I couldn't yet make 100% sure that this is how it happened, but it seems to be the only possibility so far, since my phone is always keylocked, when I carry it in my pocket. So I figure the home button was accidentally pressed, and thus the voice command was activated. Will try to investigate this further...