A method for providing emergency services directory number registration assistance for a portable Internet Protocol, IP, communications device, the method comprising: (a) storing, in a voice over IP application server, a physical IP address of a portable IP communications device; (b) receiving, at a session border controller, a registration request message comprising a physical IP address of the portable IP communications device from the portable IP communications device, wherein the session border controller is configured to maintain a mapping between a logical IP address of the portable IP communications device and the physical IP address of the portable IP communications device and to translate the logical IP address to the physical IP address of the portable IP communications device; (c) receiving, at the voice over IP application server, a registration message from the session border controller including the physical IP address of the portable IP communications device, the registration message being generated when the portable IP communications device is connected to an IP network and activated, wherein the session border controller generates the registration message in response to receiving the registration request message from the portable IP communications device; (d) determining, by the voice over IP application server, whether a difference between the stored physical IP address and the received physical IP address indicated by the registration message indicates a change in geographic location of the portable IP communications device; and (e) in response to determining that the difference between the stored physical IP address and the physical IP address indicated by the received registration message indicates a change in the geographic location of the portable IP communications device, sending a message to the portable IP communications device requesting an update of the geographic location of the portable IP communications device to update stored geographic location information for providing emergency service to a subscriber. (a) storing, in a voice over IP application server, a physical IP address of a portable IP communications device; (b) receiving, at a session border controller, a registration request message comprising a physical IP address of the portable IP communications device from the portable IP communications device, wherein the session border controller is configured to maintain a mapping between a logical IP address of the portable IP communications device and the physical IP address of the portable IP communications device and to translate the logical IP address to the physical IP address of the portable IP communications device; (c) receiving, at the voice over IP application server, a registration message from the session border controller including the physical IP address of the portable IP communications device, the registration message being generated when the portable IP communications device is connected to an IP network and activated, wherein the session border controller generates the registration message in response to receiving the registration request message from the portable IP communications device; (d) determining, by the voice over IP application server, whether a difference between the stored physical IP address and the received physical IP address indicated by the registration message indicates a change in geographic location of the portable IP communications device; and (e) in response to determining that the difference between the stored physical IP address and the physical IP address indicated by the received registration message indicates a change in the geographic location of the portable IP communications device, sending a message to the portable IP communications device requesting an update of the geographic location of the portable IP communications device to update stored geographic location information for providing emergency service to a subscriber.