It is just a matter of selecting the packet network as the access point in the SIP configuration indeed. If that does not work, but gizmo does (gizmo is sip), it means your carrier is blocking the sip ports. Try replacing the session control port from 5060 to something else. Sipphone (gizmo...) supports almost any port, but some voip providers will insist on standard ports like 5060 and 5061. Trying 5061, 5062 and 5063 for a start.