What is the difference between a gateway and a gatekeeper? Is it possible to use a Cisco 2600 router as a gateway, and if it is -- then do I need a separate SIP proxy server again for Authenticating IP phones? Can it be used to communicate between IP phones and to the telco's PSTN network?
A gateway in this context is a network device capable of converting between interface types. One type of gateway is a SIP to PSTN gateway. A gateway does not handle registration and authentication of devices. The Cisco 2600 can be configured as a SIP to PSTN gateway.
Gatekeeper is a term used in H.323 networks. A Gatekeeper handles the registration and authentication of devices. The Cisco SIP Proxy would perform this function in a SIP network. The SIP Proxy directs calls between endpoints and gateways.