Having the same problem on Intel Macbook pro with 10.5.6. Tried both NAT settings, STUN/ip-trans. Using iptel.org for SIP registrar. Trying to call <mycontact>@iptel.org and get the same message. Program is effectively useless for me because of it... was hoping to get this running with zfone as an alternative to internally-insecure skype... any replies would be much appreciated.