hi dude , i bought a 6620 in US and when i brought it outside US its not working. actully i found out that there is some GSM difference. the GSM of the phone is 850 but its suppose to be 900. so how can i cange the gsm? plss help me.

here im in armenia and they r telling me to pay the price of phone to change the GSM that is 250$.
plsss help me..