Direct Inward Dialing Saves Lines

Direct Inward Dialing (DID), sometimes known as Direct Inward Dial or Direct Dial Inward (DDI), is a technique for sharing phone lines. Everyone in the office thinks they each have their very own telephone line with their own unique number. You can print this number on business cards, publish it in a company phone directory, and otherwise let the world know it is your private number, just like at home. A thousand people at a thousand desks can enjoy this fantasy without knowing the hidden truth that there are really only 20 or so real telephone lines hooked up.

The sleight of hand involved with DID is based on a special interface between the company's PBX telephone system and the telephone service provider. With DID, the phone company reserves blocks of contiguous phone numbers for you in lots of 10 or 20 and more. What they don't do is hook up copper pairs or digital trunks for all the numbers you choose. Instead, the telco switch detects calls to your block of numbers and connects each incoming call to the next available trunk line to your PBX. You may have only a single DID trunk, a couple of DID analog trunks, or a digital PRI trunk with 23 lines available.

There's one more thing that rides along with the incoming call. It's the phone number or 3 to 4 digit extension number that your PBX system is supposed to ring. That number can be passed to your system through dial pulses, DTMF tones (touchtones), or in the data channel of your T1 PRI trunk. At that point, it is up to your PBX system to ring the correct extension and connect the incoming call.

The person calling doesn't talk to a switchboard operator or encounter an auto-attendant, so they think that they are on a direct line to the person they called. The party receiving the call hears the phone ring and picks up, none the wiser to the fact that they are actually using a shared line. When they hang up, that trunk line will be freed for the next call.

The only way this can go horribly wrong is if you run out of trunk lines due to heavy calling. If your callers can't get through, you can add more analog or digital trunk lines to handle the increased calling volume and maintain the direct phone line illusion.

The companion service to DID is called DOD for Direct Outward Dialing. This is where you dial 9 for an outside line and then dial the number you wish to call. DOD lines are shared just like DID lines. Both analog and digital trunk lines can be ordered as one-way inward, one-way outward or two-way.

Let T1 Rex help you find the lowest prices on PRI and T1 lines with DID and DOD capabilities.



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