Perhaps the scariest thing thats ever happened to me...

[Lost]

Yeh people can control your phone functions via bluetooth though, was on Real Hustle. There's a good chance the calls are going to some expensive line which the person in control owns and therefore is being paid when you make these expensive calls.

UM?! *CHECKS PHONE BILLS* :|

[Trinity]

If you have bluetooth switched on, then as other people have said, there's a chance it's someone connecting through bluetooth. Try switching bluetooth off and see if it still happens, or just do a scan for bluetooth devices and see if you can track them down/use the same tools to get revenge.

[Markeh]

Is it just numbers it seems to be pressing?

It could be dodgy keypad, possibly related to water damage

[iUnknown]

Most likely that someone in your household had installed something on your phone so that they could mess around with it via bluetooth... I doubt someone in your home was making expensive calls on your phone so that they get money....