A telephone switching device which includes a detector for identifying a call waiting (CW) signal when the incoming telephone line is in use. The detector device automatically switches the incoming telephone line to the second call and monitors for additional coded function signals such as a facsimile machine calling (CNG) signal indicating that a data transmission is desired. Appropriate switching functions are provided to connect attached telephony data devices for receiving the incoming data transmission. A microprocessor is used for receiving various function signals and performing the switching of the telephone line and attached apparatus. This allows the reception of an incoming data transmission on a single telephone line while the line is already in use. A voice message can be provided before the switching of the telephone line in response to the CW signal warning the first caller of the impending switch. Provisions are included for verifying incoming function signals and time delay sequences are incorporated for the attached data apparatus to be initiated and receive the incoming data transmission.