A signaling server (SS) comprising means for transmitting SIP signaling messages with a client (T) through a NAT address translation device temporarily binding a public address to the client's private address, including means for receiving registration messages from the client and for sending the client a validity duration, at the end of which it must transmit a new registration message. The invention resides in the fact that if the client is located behind an address translation device, it determines an approximate expiry time for the temporary binding by successively sending test messages after an increasing wait time until the termination of the binding is detected. This approximate time is then used by being transmitted as the SIP validity period.