DISA (Direct Inward System Access)

The DISA, Direct Inward System Access, application allows someone from outside the telephone switch (PBX) to obtain an "internal" system dialtone and to place calls from it as if they were placing a call from within the switch.

DISA plays a dialtone. The user enters their numeric passcode, followed by the pound sign (#). If the passcode is correct, the user is then given system dialtone on which a call may be placed.