A transcoder of a switching node MSC of a core network is inserted into a connection call of a transcoder-free-operation on the basis of an answer from a switching node RNC of a radio access network against an inquiry about a setting information of transcoder performed by the switching node MSC in a user layer of a Iu interface to insert a tone signal, etc., into the connection call of transcoder-free-operation. The insertion of transcoder into a connection call of a transcoder-free-operation can be executed without performing a re-initializing procedure.