For Scalability and ease of management, the decision has been made to centralize the location of all SIP endpoints in servers.
When phone A wants to call Phone B. it asks Testking A how to find Phone B.
What kind of device is Testking A?

A.
Proxy

B.
Redirect

C.
Registrar

D.
User agent client

E.
User agent server

Explanation:

SIP ServersSIP servers include:
1.Proxy server-the proxy server is an intermediate device that receives SIP requests from a client and then forwards the requests on the client’s behalf. Basically, proxy servers receive SIP messages and forward them to the next SIP server in the network. Proxy servers can provide functions such as authentication, authorization, network access control, routing, reliable request retransmission, and security.
2.Redirectserver-Provides the client with information about the next hop or hops that a message should take and then the client contacts the next hop server or UAS directly.
3.Registrarserver-Processes requests fromUACsfor registration of their current location. Registrar servers are often co-located with a redirect or proxy server.
Redirect server: A redirect server is a server that accepts a SIP request, maps the address into zero or more new addresses and returns these addresses to the client