·It will installed in the telephone, the user can choose any telephone lines from different telephone operators.
·When user calls, the phone line is default LINE1, after dialing #, user can choose LINE2, user can answer the calls from both LINE1/LINE2.
·If one outside line is on, the other one will play the busy tone.
·There is no external power supply, when it is installed, it can be used.
    There are two outside line interfaces, one telephone interface.