Internal numbers are private numbers that can be dialled by any connected extension in your phone system. You can then route an internal number to any destination in your account, such as an address contact, or a voicemail box. This is useful for scenarios including setting up a speed dial to commonly called contacts, or for transferring callers to specific destinations in your account such as a conference room.
When you first visit this page, you'll see list of all currently configured internal numbers in your phone system:
Each one will show the number that can be dialled, as well as the configured destination.