Over-arching thing first: If you are going to start your extension numbers with ‘1’ you’ll need to adjust several functions in the system to not use “*1”. I would move your extensions to the 300, 500, or 600 range to make life simpler later on.
Now, on to your problem. This request comes up several times a year, and I don’t think any three people have solved it the same way.
One of the first 'hurdles" is setting up the “secret” manager’s phone. There’s this article: Hooking for fun and income which may lead you to some interesting ideas.
After that, there are lots of recipes for success on this subject. One of them is to set up a queue with the secretary and the boss as static member, but set the ring-strategy such that the secretary’s phone is always the preferred one. This actually has a couple of interesting side benefits, including not having to do anything if she’s away from her desk and (if she’s already on a call) the boss can answer directly.
Of course, there are a hundred ways to skin this cat, and the forum is full of examples and discussions. If this one doesn’t strike your fancy, perhaps one of those other will.