Direct Inward Dialing (DID) and VoIP
What is DID?
Companies have the unique problem of having a lot of people working in a constricted space at the same time. Even though everyone has a mobile phone these days, it's still important for employees to have office phones through which people contact them. At home, there's mostly just one phone for the entire family and therefore just one physical phone line. When there may be hundreds of people in a building, it's simply impossible for every person to have their own physical telephone line.
What is needed therefore is for the same set of phone lines to be able to service many people. If you have a PBX system, it's easy to set it up so that each phone has an extension. Customers call a standard number and the operator (manual or automatic) directs them to the person they want to call. This solution works well for a lot of firms, but others want something more direct. They don't want to go through an operator and require each person to have their own telephone number. This is called Direct Inward Dialing or DID
How it works and relates to VoIP
To achieve this, a phone company allots a range of numbers to a company. These numbers will differ only in the last few digits. So when someone makes a call to that firm, the carrier will send only those last few digits to the firm. It's understood that the initial digits were dialed. After that, it's up to the company to take those last few digits and route them to the correct employee just as if it was connecting to an extension.
With VoIP, DID is important if you want your IP communications infrastructure to talk to the PSTN system. Having a DID number is like having an address. You can contact people and they can contact you. As a customer, you receive your numbers from a VoIP provider who provides you with hosted PBX phone solutions. They in turn receive a range of numbers from the telco and when they receive an incoming call, they convert it into a digital format and route it to you.
Building your own VoIP system means you will have to configure your VoIP server to do the same - that is extract the last few digits from the telco and send it to the correct employee. This is part of the set up of a VoIP system and is one of the reasons to switch to hosted PBX solutions instead. They do all the difficult work for you and you just have to sit back and manage your bandwidth.