DID – Direct Inward Dialing (also called DDI in Europe) is a feature offered by telephone companies for use with their customers' PABX system or other purposes, whereby the telephone company (telco) allocates a range of numbers associated with one or more phone lines.Each subscriber will be able to reach you by dialing the same number as a regular long distance number - from any phone, including and the cell.
Typically a DID is used to provide local telephone numbers for customers on a worldwide basis, or to provide access from regular telephones to non-PSTN destinations such as VoIP.
To access VoIP handsets or softphones, DID's are connected via a gateway to the trunk of the public switched telephone network (PSTN) and the organisations own internal VoIP network. This gateway connects and manages the calls between the two networks. Calls that originate in the VoIP network will as look though they have come from one of the assigned DID numbers.
There is Local Loop Operator/Telecom Operator in particular regions that transfer the calls to their required destinations.
These operators are assigned certain range of phone numbers (like 1251000 ~ 1259999) and from these numbers, Telecom Operator reserve some numbers for DID (Direct Inward dialing) service/Virtual Phone Service such as 2257000 ~ 3258000. Any subscriber, from anywhere can call on these virtual numbers
How virtual phone number work? - Let's say your business is located for example in Moscow and have representation in the U.S. For example, in this case it is advisable to buy a DID number in the U.S. which will make free calls to your employees or clients of the firm, and you will receive free phone calls in his office in Moscow from a VoIP Service or DID provider. To receive calls, you can configure any SIP device or use Skype to receive incoming calls from your DID numbers.