How do they do it & is there any site provide such phone service? read full

dear friends,
Recently i was just going thru some of shopping & classified sites like alibaba and rediff locals, i found if i wanna to speak to vendorer from alibaba or advertiser from local ads ,have to enter my number to their site and with in few sec i m getting call saying call from alibaba or redif asking to wait to connect that to advertiser or seller and then getting connected .It means i dont have to spend my phne bill and directly can speak to main person.

this was scene now what i want to know is how did it happen i just entered my number got connected to other person thru net what kind of technolgy it is and is there any site such service where you can get connected to other person???.

Thanx in advance