What are DID numbers?
Direct inward dialing numbers (DIDs) are virtual numbers that allow you to route calls to your existing telephone lines. DIDs were developed in order to be able to assign certain employees a direct number, without requiring multiple physical phone lines. One DID number is able to route and process thousands of simultaneous calls.
Direct inward dialing was created as a way to reuse a limited number of physical phone lines to calls to different published phone numbers. A company can use one DID to route all of their inbound calls.
In order to have an active DID, all that is required is to have a working transfer line to which calls can be forwarded.