Some Info about linphone
Go -> Preferences -> 'sip' tab
In the 'Identity' Area
Your sip address
Click Add proxy/registrar
Sip Proxy (may not be required)
In the main linphone tab
In Sip Address Enter
In this case the software was installed on two Ubuntu machines. Both machines are on the same subnet and use link local addresses to talk to eachother. There is no reason why this cannot function with global addresses accross the world. When I tried this in April I think it worked but it appears not to work now - I think it is because Ubuntu has further killed itself.
First thing to check is that the "Use IPv6" option is enabled in the properties of each copy of Linphone. Your properties should look as follows:
To make a call you need to enter the IPv6 address of the machine you want to call as follows:
The system will then call the client which should begin ringing. You can answer the call on the client device and start talking.