When configuring SIP extensions as remote extensions at a remote site (e.g. home ADSL), if the Com.X is located behind a NAT/firewall on the network, configure the NAT/firewall to forward UDP port 5060 to the Com.X internal IP address. If required (no NAT available), also forwards RTP ports 10000-20000 to the Com.X internal IP address.
You may also need to configure the external address on the Com.X by selecting Start -> Configuration -> Setup and then setting the External address. Configure this field to the dynamic DNS name or the FQDN that resolves to the external (public) address of the Com.X (i.e. the external address should be set to the same as will be programmed into the remote phone).
If you are using dynamic DNS, make sure that the External Is Dynamic field is set to true.