What must the virtual extension of the slave match in a 3CX configuration?

In a 3CX configuration, the virtual extension of the slave must match the master side virtual extension number. This ensures that the slave extension properly registers and communicates within the 3CX system. The master-slave relationship is critical in scenarios where features such as call forwarding, voicemail management, or call handling are shared between the master and slave extensions.

Matching the master side virtual extension number allows for seamless integration and functionality between the two extensions. It facilitates correct routing of calls, accurate reporting, and management of features associated with the master extension, thus maintaining the intended purpose of the setup.

The other options do not capture the essential requirement for the virtual extension relationship. The hardware number relates to specific hardware devices, the ISP assigned number is linked to internet service provisioning, and the user's registered phone number does not pertain to the internal configuration necessary for the extension's operational role within 3CX.