First thing you need to know is country code 381

In Serbia there are three Mobile Operators :

examples :

MTS uses 064 and 065 prefixes - 064 123456 or 065 123456

Telenor uses 062 and 063 prefixes - 062 123456 or 063 123456

VIP uses 060 and 061 prefixes - 060 123456 or 061 123456

Calling cards of all listed operators you can buy in every tobacco shop, general shops with sticker of mobile operators.