The BSC NAT/MUX makes NAT traversal easy. The real BSC is opening one TCP (or SCTP) connection to the multiplex process. The multipex process is rewriting SCCP source local references (as multiple BSCs could be connected) and is listening on UDP port 2427 (MGCP Gateway) and is forwarding the MGCP messages via TCP (or SCTP) to the BSC that should handle the endpoint.