What is an Extension Number?

An Extension Number is a short, internal phone number assigned to a specific user, device, or department within a business's private telephone system (PBX). Its primary purpose is to simplify and streamline communication inside an organization.

Instead of dialing a full public phone number, employees can reach a colleague by simply dialing their unique 3, 4, or 5-digit extension.

With the adoption of VoIP and Cloud PBX systems, the concept of an extension has evolved significantly. Key modern characteristics include:

• Cost-Free Internal Calling: Calls made between extensions are routed over the company's IP network or the internet. This means they are typically free of charge, which generates substantial cost savings, especially for businesses with multiple offices or a global workforce.
• Location Independence: An extension is no longer tied to a physical desk phone. In a modern cloud communications platform, an extension is assigned to a user and can be configured to ring on multiple devices simultaneously—such as a desk phone in the office, a softphone application on a laptop, and a mobile app on a smartphone.
• Unified Identity: This setup ensures that an employee has a single, consistent internal contact number, making them easily reachable regardless of where they are working.

Previously, connecting a remote worker's physical phone to the office PBX was known as creating an "off-premises extension." While the term is still occasionally used, this practice has been largely superseded by the superior flexibility and user-friendliness of modern desktop and mobile communication apps.