Local telephone companies offer a service to connect a customers’ private branch exchange (PBX, responsible for switching the calls to the appropriate extension) system to the PSTN (Public Switched Telephone Network, or the traditional telephone network, which does not use the Internet during a phone call). It is called Direct Dial-In (DDI) or Direct Inward Dialing (DID), both are explained in a later paragraph.
It is made on an external VoIP (Voice over Internet Protocol, a protocol, which defines the transmission of multimedia sessions via the Internet) phone, and gets transmitted to the PBX of your company, where it is switched to the desired local extension.
If an internal call is made, however, then the call is simply transmitted from a VoIP phone to the PBX, where it gets switched to the appropriate extension within the company. This process takes place entirely in the local network.
DID numbers are connected to a SIP (Session Initiation Protocol, which is responsible for making, terminating and maintaining the connection between two conversation partners) gateway that connects them to both the PSTN and the Internet. Calls between the two networks are routed and translated for the VoIP user by the gateway. If a call comes from the VoIP network PSTN users will see it as coming from one of the assigned DID numbers. As the DID gateway is not always in the same geographic area as VoIP systems, those systems are frequently called virtual DIDs.
The customer is provided one or more trunk lines to connect to the customer’s PBX, which allocates a range of phone numbers to this or these lines and forwards all calls to them via the trunk.
No operator or attendant is needed as calls are presented to the PBX with the dialed destination number so the call is forwarded to the desired extension of the organization. Direct inward call routing is available to each extension while only a small number of subscriber lines are needed to satisfy the average usage.
Eg.: If your office number is 9876543XXXX (the X’s stand for the extension number you want), then you will need to call 98765431987 if you want to call someone with the extension 1987.
Sometimes, numbers provided by the local exchange service provider. To stay with our example the given DID number would be 987 6543 1987. However, only the last 3 digits (987) would be given by your local exchange service provider (telephone and telecom companies). The PBX will complete our number with the digit 1.
Sometimes not even the three digits are provided. In this case, the PBX has a lot harder job to do. Let’s say the extension numbers vary from 8700 to 8900, but the service provider gave you DID numbers from 100 to 300. Then if somebody would call extension 219, he or she would have to dial 987 6543 8819, which only the PBX would know as extension 219, as it would need to change it 8819.
These will not be problems to the Ozeki Phone System XE, because it guarantees you will get a system with the easiest use, so even though the previous paragraphs might have discouraged you, you should know that all of those intricate procedures will be done by Ozeki Phone System XE, you will not even see any of it, except for its excellent results.