Hi experts!, please help me!!

i have an Iphone 2G jailbroked. It was working perfectly during months, suddenly today I can't make phone calls.

I can receive calls and send/receive SMS, but if I try to make a phone call the phone says "Call ended".

I am not sure but i think I had the same problem mny months ago and somebody in this blog told me some direction to change something in settings, but i dont remember what.

Please help friends!!!