Ax520 has 2 line jacks and one phone jack. The line jacks can connect to PSTN line or VOIP line. Whenever there is a ring from either of them, the phone can ring and answer.When  the  user  wants  to  make  a  phone  call,  it  goes  through LINE2 jack automatically.  If user dials ## before the telephone number,  it  will dial out from LINE1 jack.It is ideal for FXS VOIP units. Sometimes the  users don't  have  enough line  port on  PBX or KTS.  It  can  expand  the  number  of CO line ports.