The present invention relates to a flame retardant thermoplastic resin composition that contains a phenol resin derivative having good char formability, regardless of the base resin. The engaging component is made up of a first arc part disposed on the earpiece side, a second arc part that is larger than the first arc part and is disposed opposite the first arc part, a first linking part that links one end of the first arc part and one end of the second arc part, and a second linking part that links the other end of the first arc part and the other end of the second arc part. The sending device-receiving device pair may execute a training session to determine the skew compensation.