Im not sure in Unity connection, but in Unity it is called the Alternate Contact Number. In each subscriber, in the caller input you can select one of the numbers to be the Alternate Contact Number. The subscribers can input their own number in (adding the 9 to get out). If there is not an option to configure this in Unity Connection, it's not possible.
If you have some high profile people, you could configure a call handler that forwards to their cell phone. (for a few people)