The "Ring my Mobile" feature is a feature that allows users to receive calls made to their extension on their mobile phone, in addition to any other devices that are set up to receive calls, such as a desk phone or headset. 

Alternatively, an office worker who is often away from their desk for short periods of time may find it useful to have their extension calls ring on both their desk phone and their mobile phone. This way, they can answer calls on either device, depending on which one is more convenient at the time.

On the other hand, a simple call forward means that calls made to the user's extension will only be forwarded to the user's mobile phone, and will not ring on any other devices. This means that the user will only be able to receive calls on their mobile phone when they are not at their desk.

The "Ring my Mobile" feature can be a useful tool for businesses that want to ensure that their employees are always available to take calls, even when they are not at their desk. It can also be useful for employees who need to be able to take calls while on the go, such as sales reps or field technicians.