Linphone is simple to use.
Linphone, as all SIP-compliant phones, uses a SIP address to identify the destination of the phone call.
A SIP address is something that looks like closely to a SIP address, for example sip:alice@example.net.
The domain name (example.net) can also be directly an IP address.
To call someone that has another SIP phone connected to the internet, just type his SIP address in the sip address bar, then click "Call/Anwer". You should then hear a remote ring, and once the person you're calling has answered the call , you 'll hear and (perhaps) see it.
But very few people have actually a domain name for their computer. That's why there exist SIP proxies in the internet with which you can create for free a public sip address.
A list a free SIP proxies is available below.
The proxy will also help linphone to pass through NAT (firewalls) whenever possible, that's why it is always recommended to use a proxy.
First follow the instructions given on the website of the registrar you've choose. You surely need to register and choose a user and password.
Then go in the property box of linphone, in the SIP tab, and click "add proxy". A popup window appears, then enter the fields as below.
Supposing you have registered to "myregistrar.net" with a username "alice",
server address: sip:myregistrar.net
identity: sip:alice@myregistrar.net
send register : toggle yes
Other fields are optional.
Click OK and you're done. Now people can join you by calling sip:alice@myregistrar.net !