A virtual extension allows the control of Hosted PBX features such as call forwarding, find-me/follow-me, and voicemail for an extension that is not associated with an actual desk-phone. As such it has a lower monthly rate than a full Hosted PBX extension.
This can be useful for employees that don't spend a lot of time at one desk or may not spend much time in the office at all. By assigning virtual extensions to these employees, customers and co-workers will be able to reach them easily and transparently. These virtual extensions can be reached the same way as a normal extension who is in the office at their desk, simply by dialing their short extension.
If a 10 digit phone number (DID) is associated with the extension, than other cell phone using employees can use the contact list on their cell phone to get to your employee in a way that supports call logging, voicemail and find-me/follow-me.