A DID (Direct Inward Dial) number is a virtual phone number that is used in a Voice over Internet Protocol (VoIP) system. It allows a user to receive incoming calls from the public switched telephone network (PSTN) or a private phone network. The DID number acts as a routing number, directing the call to the designated VoIP phone or device. It can be used to set up a virtual phone system, allowing users to make and receive calls from anywhere with an internet connection. DID numbers are commonly used in business settings to provide a professional appearance and allow for flexibility in communication.