2 telephone lines interface

hello

connect two tel lines to one modem so when ring on first line (modem answer on first line), second to be ocupied.

first idea was to connect reed relay to first line and when detect ring second line to be shorcircuit on 300 ohm, like is in use, but i need to bee shortcircuit whole time while convcersation on first line >

Any sugestion , Thanks in advanced