A multiple line modem and method enable a user to automatically usurp a plurality of telephone lines for data transfer when the telephone is not in use and the ability to switch from multiple line data operation to one (or more) line data and one (or more) line telephone operation automatically when the telephone handset is lifted or an incoming call is detected signaling a request for voice service. The multiple line modem will automatically reestablish the data connection on the line(s) that was used for voice service when the voice service terminates. The multiple line modem allows end to end service that is transparent to the central office. When all lines are operating in data transfer mode, the aggregate data transfer rate is multiplied by the number of lines available for data transport. When one line(s) is operating in voice service mode, the other line(s) maintains data transfer operation at a reduced rate. The line(s) that is used for voice service is adaptively managed by the multiple line modem to provide seamless switching between data transfer mode operation when voice service is not requested and voice service mode operation when voice service is requested.