AX520 has 2 line jacks and one phone jack. The line jacks can connect to PSTN line and 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 LINE1 jack automatically. If user dials #,0 before the telephone number, it will dial out from LINE2 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.