Integrating VOIP with my existing phone system

Hi,

I have a PABX phone system which has 3 Phones connected to our section of the office  i wish to add a VOIP box into the system so they can use the existing Phones and get the benefits of the cheaper VOIP rates.  I am unsure if i can do this, if anyone can tell me how to do this it would be great.

I would like all 3 phones to be able to pickup there existing phones and route through VOIP for outgoing calls and still receive incoming calls through the existing PABX system. how do I set this up, do i need 3 phone plugs at the router ????????

thanks

Big_Daddy