I used Stanacard. Its the best card I've ever used to call Nepal.Cheap price and excellent quality of voice. No connection charge, no hassle of pin no. You can dial direct number. 

Also there is provision of bonus credit system. If you buy credit of 10 dollar using this link

http://www.stanacard.com/?ref=254194  , you will get another 10 dollar bonus. So enjoy guys...