You can call your friend from your computer ( PC ) to his mobile handset free of cost if Skype (Mobile version ) is installed on his mobile phone.
As you know that, you can call free of cost from PC to PC by using Skype software ( Similar to yahoo voice chatting ).
But if you try to call to normal mobile phone ( Skype is not installed on it ) from your PC through Skype account, it will charge you. But if the mobile phone has skype installed, the mobile phone will act like PC for Skype. The only requirement is that mobile should be GPRS enabled. Skype account logged ON. See this
Suppose, you are using computer now and you want to make a free call to your friend Tom. You should know the Skype ID of your friend. Tom is required to having a Skype installed mobile hand set. GPRS working, and skype is ON.
You should make a call at your friend’s Skype ID (don’t use cell number). Your friend will hear the ring from skype of his mobile, after that he can talk to you. The only cost is GPRS activation charge for mobile handset. Most of the people always using GPRS ON for other uses ( its cost is negligible).
Reverse process is not possible. i.e your friend Tom can not call from skype using his mobile to your account.
So, by using this method you can use skype to get almost free VOIP service in any country.