There are many VOIP service providers ...
Here is one of the such provider with best calling options..
There are 3 ways to call...
Using traditional PC dialer :
You need high speed internet connection.
It is easy to use and has features like address book, call history, etc
Phone to Phone :
Enter your number and the destination number and press 'CALL NOW'
You will receive call back from ACTION VOIP and connects you to destination number.
However, you will be charged for both calls.
From your Mobile / landline :
Add your mobile / landline number to ACTION VOIP personal details and call the local access number (local rates will apply) and dial the destination number.