These are the same people who send those bogus emails telling you that you have won a large sum of money. It seems that those emails don't work anymore so they have shifted into calling numbers overseas. As han19 said, just don't return the call because it will encourage them to call you again. What I usually do when I'm not familiar with the country code of the incoming call is to check the internet for the country which uses that code. When it shows that it's from a country where I don't have any known relatives or business interest, I just ignore the call.
@quiou, I got some of those calls too but I didn't check if some amount was deducted from my credit. But if we are deducted 3 riyals for each of those pesky calls, then it's not fair.