My ISP gives me 2 free phone numbers to use for SIP. Gizmo and Fring work for me even without STUN, but the build-in SIP client in Presence just doesn't want to work. After trying to call, immediately disconnected. It doesn't even work with STUN. It only works when I'm on a neighbor's WiFi. Then it works fine although laggy. I wonder what I'm doing wrong...? Gizmo and Fring are too Teletubby & proprietary for my liking.