Bobby Joe, I Supose that you know how to use the command line interface and your A800 is loaded with lastest firmwire (with SIP/H323)
so 1 st goto config then type :Unit 1
after that type : (to use SIP) then
goback to the config root by typing Exit (2 times). You need to save the so type :
2nd goto SIP (from config line )and type: you will see the command to setup the sip Proxyserver ip and user/pass ect …
Hope this help