How do I forward calls to another extension when calling an outside line?

Question

VoIP

Hello, help with setting up Cisco Call Manager Express.
The head has the number 100, and the secretary 101.
How to make sure that the head could not be reached from city numbers?
How to make it so that when dialing her number, from an external line, calls are redirected to the secretary? Moreover, if employees call on a short number, then they should go directly to her.