DISA= Direct Inward System Access, used to access the system and make calls based on NCOS, TARG/TGAR and PWD. As firebirdscrambler said, this can be used by Hackers to access your system and make LD calls. If programmed correctly with Passwords, AUTH Codes, NCOS and TARG/TGAR. If it's not being used by anyone it should be remove for Security reasons.
This is a Signature and not part of the answer, it appears on every reply.
This is an Analogy so don't take it personally as some have.
Why change the engine if all you need is to change the spark plugs.