VOIP routers aren't openly sold at computer stores in Qatar. However, I can let you know few shops at Sofitel that sell them. 

Basically what Umsarmad was talking about is prepaid calling cards you get at shops at Sofitel. The shopkeepers bulk credits in bulk and sell them to end users like you at smaller denominations. It is slightly expensive than buying it online by yourself, but saves you from the risk of exposing your credit card details online. I'm not trying to say that Yahoo voice or Skype is risky, they're pretty safe and I've used them before to buy credits when I didn't have VOIP router installed.
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E4U Presents
"DJ Aqeel - Live in Qatar"
@ Qube, Ramada Plaza Doha on April 4, 2008
http://www.e4u.name.qa/main.htm
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