DID: definition and a practical example
Short answer: DID (Direct Inward Dialing, called DDR in Brazil) is the service that maps public numbers to the extensions of a PBX, so outside callers reach an extension directly without going through an operator or an IVR.
In Portuguese: DDR
What is DID?
DID (Direct Inward Dialing, known in Brazil as DDR) is the carrier service that gives a company a range of public numbers, such as (11) 3456-7000 to 7099, and lets each number reach an extension on the IP PBX directly. Someone who calls (11) 3456-7012 talks to extension 7012 without going through the front desk or the IVR.
An extension is the internal number of each phone, softphone or group inside the PBX. The DID is the bridge between the public number and that extension.
The DID range reaches the company over a SIP trunk or an E1 trunk. On each call, the carrier states which number was dialed: in SIP, in the Request-URI and the To header; on E1, in the called (B) number. The PBX reads that number, usually the last four digits, and looks up the matching extension in the dial plan.
The most common failure lies in that agreement on digits. If the carrier changes the format it sends, the PBX stops recognizing the number: the call falls to the default rule, lands on the operator or is rejected. The trunk stays up, the extension stays registered, and only a call from outside reveals the problem.
In practice
A 100-number DID range with the PBX reading four digits:
| Number dialed | Digits received | Destination on the PBX |
|---|---|---|
| (11) 3456-7000 | 7000 | Front desk IVR |
| (11) 3456-7012 | 7012 | Extension 7012, finance |
| (11) 3456-7050 | 7050 | Support group |
| (11) 3456-7012, after a change at the carrier | +551134567012 | Default rule: operator |
In the last row, nothing changed inside the company, yet finance stopped receiving direct calls. Internal calls to 7012 keep working, which delays noticing the problem. That is why the test has to come from outside, through the public number.
How Monitor uses this
Any dialable number can be monitored, including extensions reachable by DID. The test dials the extension’s public number, just as a customer would, and goes through the carrier, the trunk and the PBX dial plan. See SIP trunk monitoring.
Frequently asked questions
What is the difference between a DID and an extension?
An extension is the internal number of a phone inside the PBX, such as 7012, dialed between colleagues. A DID is the public number that leads to that extension, such as (11) 3456-7012, dialed by people outside. Every DID points to an extension, a group or an IVR, but not every extension has a DID: many only receive calls transferred by the front desk.
Why does a call to a DID land on the operator?
Usually because the PBX did not recognize the number it received and fell back to the default inbound rule. That happens when the carrier starts sending a different number of digits, for example the full number with +55 instead of the last four, or when the extension was removed without updating the dial plan. The PBX call log shows the number that arrived.