Are you forwarding port 5060? You may need to forward another one too, as I think there's two ports required by SIP. I would have thought the Linphone help would explain how to set up your network...? There may also be a setting in the software to tell it that it's behind a NAT router. It's been a long time since I tried to do anything like this, but I vaguely remember having to do that with the software I used back then.