Treo 750 random dialing.

I have a Treo 750 that has randomly dialed a number out of the blue.
It was number of someone I knew and luckily they have been
understanding so far. Its not just an inadvertent last number redial
cause I called my self and it still called this person after that. I
do have the key pad lock on, but I dont lock the phone with a
password. I do have a full case with a snap over type lock.
In the process of trying to fix this I did end up locking out or
erasing my sim card and had to stop by ATT to get it reset.
Has anyone ever heard of this? I don't use that many minutes per
month, but one of these calls lasted about 20 minutes. That's 20
minutes I didn't plan on using and really didn't use them.

Greg