First check that the gateway is being pointed at the correct SIP proxy and the correct port (see above). If that is correct then check for any denies in the firewall logs for SIP traffic. It is possible that there might be unintentional blocking of UDP 5060 traffic in the firewall to / from the SIP proxy server or there might be an access-list on the edge router that is not allowing the SIP traffic to pass to / from the gateway. If the IP-PBX Gateway is behind a NAT and the gateway is not registering with Switch2Voip, check the NAT rules in the router to make sure that the SIP traffic is reaching the private network from the public network. If possible, check the SIP logs on the gateway to see if you are getting any SIP replies from Switch2Voip.