Method for optimizing the reduction of mobility signaling after an Inter-RAT change for a UE roaming between a source area covered by a source telecommunication core network and a target area covered by a target core telecommunication network, wherein a signaling reduction function is activated by the source telecommunication core network, and wherein deactivation triggering events of the signaling reduction function generates a need for registration update at next IRAT change occurred. The method comprises the steps: storing in the UE a parameter the value of which indicates whether or not the deactivation triggering events of the signaling reduction function only correspond to the UE context modification, and, after the Inter-RAT change, checking the event generating the Inter-RAT change, and, checking the value of the parameter, cancelling the need for registration update or validating the need for registration update depending on the event generating the Inter-RAT change and on the value of the stored parameter.