There is disclosed a communication system, and an apparatus therefor, for use in combination with an integrated service digital network. The proposed system, and the apparatus, can achieve a change of service from an equipment to another, for example from a telephone unit to a facsimile unit, both connected to a same telephone line, without disconnecting a call made through the telephone line. For this purpose, the proposed system or apparatus has a discriminating unit for identifying a request for communication released from another unit connected to the same telephone, and a communication control unit for automatically commu­nicating with a destination unit with which said another unit has been communicating through said telephone line.