Linphone is a Linux softphone built using GTK2 libraries.
To configure linphone to use voipuser's community outbound service, go to connection->preferences->SIP. Use the following configuration:
*SIP Port: 5060 (default).
*Identity->SIP address: username [!at] voipuser.org (replace the [!at] with a @) (where username is your voipuser username).
*Mark "Use SIP registrar".
*Server address: SIP:voipuser.org
*Password: you voipuser password.
Here's an example for my user:
Now go to "codecs" tab. By default, connection type is set to "56K Modem". You should change it to "ADSL or Cable" ir order to change the codecs to use. You can now set your prefered codec, but be warned that with voipuser community you can only use G711u or GSM. For example:
With this configuration linphone will use PCMU (G711 ulaw) if possible. If not possible, it will use GSM, and so on.
One more thing: if you dial a number like "0034912345678",
it won't work. You have to dial something like "0034912345678@voipuser.org". The same applies when you configure your agenda (connection->agenda). Probably a bug in linphone.