VOIP 101:Gatekeeper is a device (hardware or a PC plus software)that let a PC or a VOIP device find the called device/PC.
On the most basic form a gatekeeper have the I.P. addreses of the devices that are on line, and when a new device logs in and “ask” for a particular device, the gatekeeper inform the calling device of what is the I.P. of the called device. The originator then calls the receiving one by its I.P. address.
Gateway:Under control from a gatekeeper, this device lets you hop-off the data world into the old PSTN services.Act as a bridge from data to voice.
There are many free softphone on the Web. One of the most common ones is netmeeting.That will allow to call anyone with neetmeeting or similar programs,PC to PC.
To call PSTN lines you need to use a gateway or pay per minutes charges to someone like Net2phone or similar company.
About geting a gateway or gatekeeper. There are some units out there bellow $1000.00 dollars.
A small two ports gateway could be found for about $400.00 and a combo gateway/gatekeeper with four ports/lines goes for about $800.00
A basic ATA (two telephone lines to ethernet adapter) could be around $200.00. (Cisco ATA 186)