make phone call to us and user through web site

Question

hi
I am developing the web site with C#.Net and SQL server2005.
In this web site I need one functionality to create free phone calls.
There is one link for user, when user will click this link, he will get the form, where he will enter his name and phone number.
when he enter all information and click on make free call link. phone call goes to us and him too.
Same functionality we can see on the below link.
http://www.indiamart.com/
On this site go to any service and in contact detail there is an option of make free sms/call.
Thanks,