we can call a number that is not registered with CUCM.

[JustTakeTheFirstStep]

If you dial 2307 from your office phone, the call is connected.
actually using 2307 for the branch.
2307 is not a registered number with CUCM.
Why can I call 2307??
In CUCM?

[Roger Kallberg]

Have you checked in DNA what the path is for a call to this number?

[Jaime Valencia]

CUCM comes blank when it's installed, all the dial plan has to be configured by the admin.

If you're able to dial a certain DN/pattern in CUCM, is because your CUCM is configured to route it.

So, you can call 2307 because there is something in your dial plan that matches that number.