Registering on a SIP server can be usefull in two main cases:
Your machine does not have a public domain name, which prevents other users to call you as they can't guess your IP address. In this case, you can register to a proxy or redirect sip server to get a public sip address. For example, you are <sip:bob@no-host-name> and let's suppose it exists a redirect or proxy sip server at <sip:myserver.org>. By registering to <sip:myserver.org>, your friends will be able to call you at the address <sip:bob@myserver.org> . The proxy or redirect server myserver.org will forward or redirect the calls from your friends to your exact location.
You are behind a firewall. In order for UDP messages (SIP and RTP are on top of UDP) to go through the firewall correctly, in most cases a SIP proxy running on the firewall machine is necessary. You must indicate to linphone the proxy to be used, and tell it that this proxy is an outbound proxy (ie all messages must go through it).
In order to setup the correct registrar (a registrar is a proxy or redirect server) configuration, you have to indicate the registrar url in the SIP section of linphone's property box. Do not modify the field “address of record” unless you exactly know what you are doing. If the registrar has to be used as an outbound proxy (if you are behind a firewall for example), then toggle the “use it as an outbound proxy line”.
Finnaly click the OK button of the property box. Linphone's property box will close and immediately a message will be sent to the registrar to inform it of your exact location. When linphone shutdowns, it will take a few seconds to unregister your location from the registrar.
A list of public registrar servers can be found at http://www.cs.columbia.edu/~hgs/sip/servers.html.
Unfortunately, many of these servers don't work anymore, maybe caused by the recent crisis in telecommunications. Some other require authentification methods that are currently not supported by linphone.
In order for you not to lost your time, a list of working public sip servers usable with linphone is availlable on the old linphone 's web site at http://simon.morlat.free.fr/english/servers.html.