DISA (Direct Inward System Access) allows someone calling in from outside your office to obtain an "internal" system dialtone and dial calls as if from one of the extensions attached to the telephone switch. Basically, you call into a back line of your phone system witch invokes the DISA application. The DISA application in turn requires the caller to enter a passcode, followed by the pound sign (#). If the passcode is correct, the caller will hear dialtone on which a call may be placed. The call can optionally be set up to record by default.
Why would you want to do this? Let's say you need to call a customer quickly but you're out of your office. You can use your cell phone to call the customer by dialing into your DISA application to access the VoIP phone line from your cell phone. When your customer sees the inbound call, it won't show your personal cell phone number, but rather your office number.
The DISA technology allows our Cell Phone Bridge feature to work based on inbound Caller ID.
This is a standard feature on our Cooler Master PBX.