A first media packet from a first endpoint of an access network behind a NAPT device is received by a media device between a core network and the access network. The first media packet includes a first source IP address and port combination identifying the first endpoint. An UPDATE request or a reINVITE request is transmitted by the media device. A second IP address and port combination for the media device to receive future media packets from the first endpoint is negotiated. The media device compares a first IP address of the first source IP address and port combination to a second IP address of a second source address and port combination for a second media packet received on the second IP address and port combination. If the first and second IP addresses match, the media device relays media packets from the core network to the first endpoint.