You must choose a network interface to use with linphone. If you want to contact somebody on the internet, you should choose the network interface that connects your computer to the internet. For example, if you are using a 56k modem, it should be interface 'ppp0'.
If you are not connected to any network, only the local interface called "lo" will appear in the list. The only thing you can do in this case is to call sipomatic.
Select here the way you are connected to the network you want to use (in most case the internet). This help linphone in configuring itself according to the bandwidth of your connection type.