Sounds to me like you have not set up any SIP account in the application, from which to call out.
Go through the following process and report if anything does not work ..... first undo any changes you may have made to Linphone settings ....... completely remove and then install again if unsure. Delete the file
~/.linphonerc . This should ensure a clean start.
Go to
https://www.linphone.org/eng/linphone/register-a-linphone-account.htmland create a new user account --- a free SIP account.
I created an account called 'pclos', received my confirmation email, and clicked on the link in the email to activate the account.
That brought me to a page which confirmed activation and gave me the 'set up' info needed for Linphone or other SIP client. Copy those details to a text file ..... you
will need them.
This is what I copied:
Sip identity : sip:pclos@sip.linphone.org
Username : pclos
Domain / Proxy : sip.linphone.org
OK, that is the new SIP account taken care of.
***
Launch Linphone and select Preferences from 'Linphone' on the toolbar.
Network Settings: no change for the moment
Multimedia Settings: I changed all the devices to Pulse as that is what I use, and then hit Play to test the output. For most users there will be no need to change anything here.
Manage SIP accounts: Add the account you set up, using the details supplied from the Linphone webpage and which you copied.
User Interface: Select Language. If you select to "Show Advanced Settings" then you get a Codecs Tab.
Codecs: Here you can select preferred codecs for Audio and video .... not needed initially.
Setup complete! Select Done.
When calling out, that free SIP account will be used by default.
You can add more accounts to Linphone, and select when making a call which you would like to call from.
Calls can be received on any account set up and registered.
regards.
E&OE

EDIT: for misspelling corrections