[SR-Users] i want to setup a SIP proxy and i dont know where to start.

i have a main office and a branch office. the main office and the branch office connected via vpn connection the branch office is natted to the main office. so i want to put on the gateway machine a SIP proxy server to nat the sip connection and also to proxy all the connections to SIP uses of the office. means i have one asterisk machine on the main site and other SIP providers that in a case i want to use i can use them also without connecting to the main office. i dont need asterisk on the branch office. the gateway server is ubuntu 10.04 amd64 what i should use Kamailio or the openser on the ubuntu repos? for now the Kamailio is installed.(not working). another feature i want to add is to be able to make calls between SIP phones using this server such as berni(a)mydomain.com from outside the office and vise verse... to dial people at bob(a)otherdomain.com hope for little help Eliezer