Abstract: A telephone line switching interface unit (10) connected between a telephone set (50) such as a multi-line telephone and a telephone accessory unit (60) that includes cordless telephones, FAX machines and telephone answering/recording machines. When the telephone accessory unit (60) is "on hook", an arriving telephone signal is answered normally at the telephone set (50). However, when the accessory unit (60) is placed "off hook", the control electronics, circuit elements (12)-(36), in the interface unit (10) cause the telephone signal arriving at the telephone set (50) to be automatically switched to and answered by the telephone accessory unit (60). The need for the accessory unit (10) arose from the problems encountered in connecting telephone accessories into multi-line telephones.