Trouble with phone calls - Movistar Colombia

[iruruley]

Hi, I´m using a 9550 - .1015 from Verizon here in Colombia using a prepaid account with movistar (using data by sending a SMS). I've been having a problem with the phone calls outside Cali, the city where I'm living.

The problem is:
I can make one or two calls with no problem at all, but a few minutes later the calls won´t work. As soon as I press the call button for any contact, it appears "Calling" but it stays mute and changes to "Disconnected". Sometimes while I'm making a call, I hear 6 beeps and says "call error".
The only way I've been able to fix it is by a battery removal. (but as we all know it takes an eternity)
I haven´t had any problem by receiving calls, I've proven it and the problem is only for the outgoing calls.
As I said before, the problem appears when I am in any city different from Cali.

I hope someone can help me with this. I've asked movistar but they just say that this phone is not supported by them, so they can´t help me.

Thanks!!!