How DISA enables you to use corporate PBX to make long distance calls from anywhere

Architecture Diagram - DISA - Direct Inward Station System Access
Architecture Diagram - DISA (Direct Inward Station/System Access)

Often, the professionals who are roaming might need to make long distance calls (to other countries, for example) but calling from their cell phone directly to an international number might be a more expensive option. We will see how DISA – Direct Inward Station Access helps  employees in such situations.

Let us consider the set up shown in the above diagram. There are two locations: International (above the cloud), Local (Below the cloud). We have an IP PBX in the company (corporate/ enterprise/ SMB). We have all the trunk lines terminated to this IP PBX (even the extensions are terminated here, but for our purpose, we have not considered the same) like the PSTN (ISDN, PRI, FXO), Internet (SIP Trunk, Leased Lines – via the network, etc) and the Cellular Gateway (Through the FCT – Fixed Cellular Terminal connected to the IP PBX. When anyone calls to the Mobile SIM Number which is inside the FCT connected to the IP PBX, they can reach the main auto-attendant of the IP PBX).

There is a mobile user who needs to call another mobile number, but in an international location. Normally, this becomes an expensive call especially if they are going to talk for a long time. But in our case, the mobile user, can actually call the corporate IP PBX through the cellular network and the cellular gateway (FCT). He can even call from the cell phone to the PSTN trunk number, but that would involve some cost. With the cellular gateway, the SIM card in the FCT can be a CUG number in order to make this call free of cost. Well, almost.

After reaching the IP PBX, he would be able to seize a trunk line connected to it (in our case it would be the SIP trunk over the Internet) and call the other person’s cell number and establish communication. In this case, only the local call charges (if any) of calling the corporate IP PBX needs to be borne by the mobile user. The remaining cost of calling the international cell number would be to the account of the company. Its better this way, as the SIP trunk charges to many countries are quite less and the company can negotiate for bulk usage. Even otherwise, the company needs to pay for the international call over a cellular network for the mobile employee, which is a costly option, comparatively.

How does the corporate PBX recognize that this mobile call number is coming from a mobile user of the same company? Can any outsider just call this number and start making international calls? IP PBX systems handle this in different ways. Most of them have the numbers (cell numbers and land line numbers, which would be utilizing this DISA service) pre-programmed, so that when a call comes from that number, it could automatically allow access to the trunks. Some of them can even request a password before giving access to the trunks. Some of them can play a separate IVR (Auto Attendant voice message) for these users and let them select the available trunks (the ones which they are authorized to use).

As shown in the diagram, even a local land line number (for example, CEO’s residence number) can be authorized to use this DISA – Direct Inward Station Access facility. This service can be profitable to both the companies and the employees involved as it can bring down the ongoing international call charges.

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