You can use the BLF hint BOSS_EXT, where EXT is the Boss’s Extension number, to monitor the state of the Boss/Secretary function on the Boss’s phone.
You would be able to use the same BLF hint above for the secretary.
This is a very traditional feature for phone systems, so this would be common practice for the secretary to manage the Boss’s incoming phone calls. Depending on the Business’s workflow, the secretary will tend the phone calls similar to a receptionist. I would be able to make the suggestion to the dev team, but this is how it has been traditionally handled in most traditional phone systems.