To assign extensions to users, you must first

In Genesys Cloud, before an extension can be assigned to a user, it must first exist as a configurable object within the system. The standard administrative procedure for creating multiple extensions is to define and generate an extension pool. This pool acts as a repository of available, pre-configured extensions. Once the pool is created, individual extensions from that pool can be allocated and assigned to users, phones, or other system entities like IVR menus. This creation step is a mandatory prerequisite for the assignment process.

B. Buy the extension number from the carrier. Extensions are internal to the Genesys Cloud organization's private telephone network; they are not purchased from public carriers.
C. Add the extension to the dial plan. A dial plan determines call routing for existing numbers. The extension must be created before it can be referenced in a dial plan.
D. Assign the extension to the user's phone. An extension is assigned to a user, not directly to a phone. The user then logs into a phone (physical or softphone) to use their assigned extension.

1. Genesys Cloud CX Resource Center, "Create an extension pool": This document outlines the procedure for creating a range of extensions. The overview states, "Create an extension pool to generate a range of extensions that you can assign to users, IVR systems, managed phones, or campaigns." This confirms that creating the pool is a precursor to assignment.

2. Genesys Cloud CX Resource Center, "Assign an extension": This guide details the assignment process. The first sentence of the procedure is, "You can assign an existing extension to a user, phone, or workgroup." This explicitly states that the extension must already exist before it can be assigned.

3. Genesys Cloud CX Resource Center, "About number plans": This document explains how numbering plans work, including extension ranges. It notes, "The number plan defines the valid range of internal extension numbers for your organization." This definition of the valid range (often done via an extension pool) is a foundational step before individual extensions can be used or assigned.