DID stands for Direct Inward Dialing and is a virtual number that helps users anywhere in the world to communicate with your company without having to assume the costs of a long distance call.
It is the trunk number of the reference telephone line from which all the extensions of a company are branched to different agents or departments.
This functionality is very useful for companies that use the PBX system where the telephony and data provider assigns a certain group of numbers associated to the telephone lines.