DID stands for Direct Inward Dialing. It's a feature offered by telephone companies for use with their customers' PBX (Private Branch Exchange) system. Essentially, DID allows outside callers to dial directly to a specific extension on a PBX without the need for a human operator or receptionist.

Here are some key points to understand about DID:

1. Virtual Numbers: In the context of DID, the numbers provided by the telephone companies are often called "virtual numbers" because they don't have a dedicated physical phone line. Instead, they are mapped to specific extensions or devices within the PBX.
2. Cost-Efficiency: Before the introduction of DID, businesses needed individual phone lines for each employee or department that wanted a direct number. With DID, a company can have fewer physical phone lines than direct numbers, saving costs while still offering direct access to employees.
3. Integration with VoIP: In modern telephony, DID numbers often get integrated with VoIP (Voice over IP) services. This means that even if a call originates from the traditional PSTN (Public Switched Telephone Network), it can be directed to a VoIP endpoint through a DID number.
4. Global Reach: Many VoIP service providers offer international DID numbers. This means a business can have a local phone number in another country, allowing customers from that country to call them at local rates, even if the business's actual location is elsewhere.
5. Use Cases: DIDs are commonly used in call centers, where individual agents can have direct numbers. They are also popular with businesses that want direct numbers for various departments or high-profile employees.
6. Number Portability: Many regions and countries allow for DID number portability, meaning businesses can keep their DID numbers even when switching service providers.

In essence, DID provides a mechanism for businesses to offer a more personalized and direct connection for their clients and partners without the overhead of numerous dedicated lines. It streamlines the call-routing process and can enhance the professional image of a business.