DISA (which stands for Direct Inward System Access) allows you to
provide an internal dialtone to external callers. When you configure a
DISA destination, you can use it as a menu destination within a Digital
Receptionist, so that you can get an internal Asterisk dialtone. This
means you could call into your Asterisk system and dial out as if you
were using an extension connected to the Asterisk box itself.

From the “Applications” menu, choose “DISA”.

It describes the feature as: “DISA is used to allow people from the outside world to call into your PBX and then be able to dial out of the PBX so it appears that their call is coming from the office which can be handy when traveling. You can set a destination in an IVR that points to the DISA or set a DID. Make sure you password protect this to keep people from dialing in and using your PBX to make calls out.”

Sources: http://www.freepbx.org/support/documentation/module-documentation/disa