Direct Inward System Access. A PBX feature that allows remote access to the system, generally on a toll-free basis.The caller dials the telephone number associated with a special DISA trunk. After the PBX answers the call, the caller can gain access to connected resources by entering the proper authorization codes. DISA is useful to authorized users requiring remote access to e-mail servers, voice processors, computer systems, and outgoing toll trunks. However, DISA may open the system and all connected resources to hackers, who often target them in order to gain unauthorized access to the same resources.