Direct Inward System Access (DISA) is a system feature which allows a user to access the phone system from external sources.
This feature allows users (for example, on a mobile device) to connect into the phone system in order to check voicemail, place calls, dial extensions, and more. This feature works by prompting users to first dial an applicable number (this can be your regular business number or a special number specifically for this purpose), then (this is strongly recommended) dial a special feature code which will then prompt the user to enter a special PIN code. After entering the credentials, the user has access to your system almost as if they were at their desk.
An example of this would be if an employee needed to contact a client when they are not currently in the office. The employee could dial the special number, enter the feature code (#XX), enter the PIN (XXXXX), and proceed to place a call. This is beneficial if the employee (or employer) only wanted clients to see the work caller ID number, and not the employee's personal or work mobile caller ID. Another unique benefit to this feature is that the caller can use the phone system's long distance service without worrying about paying ridiculous mobile long distance per-minute fees. In effect, DISA can work to be a calling card for employees.