Linphone uses the Session Initiation Protocol to establish the connection with a remote host. In this protocol each caller or callee is identified by a SIP url: sip:user_name@host_name. A sip url is very closed in syntax to an email address, excepted the sip: prefix.
User_name is like a login name on an Unix machine, and host_name is the name or the IP address of the machine the user can be joined.
Note that Sip is a new telecommunication protocol designed to be simple, and it is not compatible with H323 at all.
For that purpose, you can register to a sip proxy. There exists several sip proxies on the net, for example see http://iptel.org, it's free. You'll just need to get an account on the proxy and then tell to linphone to use it.
run sipomatic from a terminal. Dont't be surprised, sipomatic does not have a graphical interface, but you don't have to interact with it.
Then type the following sip url in the main window of linphone: sip:robot@127.0.0.1:5064 . 127.0.0.1 is a local address for your computer, and robot is the name to use for calling sipomatic. 5064 is the port where sipomatic can be joined. Normally you should always use 5060 (i.e the default port when no port is specified) to call somebody, but sipomatic is the exception: it runs on port 5064. The reason for this is that linphone already runs on 5060, and you cannot have two applications running on the same port, in the same time and on the same machine.
Then press the call button. After one second, sipomatic should answer to your call and you should hear a short annoncement.