There is a virus/trojan horse problem making it's rounds that is effecting several Windows Phone users. The virus came to light over at XDA Developers Forums when several affected users noticed their Windows Phones dialing international phone numbers spontaneously in the middle of the night.
In discussing this phenomena, one commonality was discovered and the likely culprit is a game titled "3D Anti-Terrorist". The .cab file not only loads the game onto your phone but also loads a trojan file on to your Windows Phone is time triggered to dial certain international numbers monthly.
If you've downloaded "3D Anti-Terrorist" you may want to check your call history for any mysterious calls happening in the middle of the night. If you think your phone may be affected by this, the solutions offered over on XDA range from a hard reset to deleting certain files. The hard reset option seems to be having the most success.
If more develops on this, we'll pass it on.