What you are saying now is completely different from your original post.
Your stated that when caller dials a number, XXXXXXXXX, to ring both the boss and assistant. When one of them picks up, the other stops ringing. This is a ring group by definition. You then wanted whichever one of them picked up to be able to connect the other with themselves and the caller by pressing a button. That is a conference or three-way calling by definition.
Now you are talking about intercepting an active call (I’m not even sure what you mean by this).
There will need to be some manual interface by the boss or assistant.
Simply create a ring group called “Boss/Assistant” and add the boss and assistant’s extensions to it XXXXXXXX1 and XXXXXXXX2.
Then create an inbound route for the desired number the caller will dial- XXXXXXXXX and set the destination as the “Boss/Assistant” ring group.
When someone dials XXXXXXXXX, the phones on both the boss’ and assistant’s desks will ring. When one answers, the other will stop ringing.
Test that this works.
Page 45 of the Aastra 6739i manual discusses Programming Function Keys. Most likely, you will need to assign the key type as Destination and then enter the feature code for a conference call on your system followed by the extension to be dialed. (On the boss’s phone, enter the extension for the assistant’s phone and on the assistant’s phone, enter the boss’ extension.
Test that the key/button works.
Finally, call the ring group number and have the boss or the assistant answer the phone. Once answered, have them press the key/button for the conference.
In this setup, no one needs to remain in the conference when not needed but the boss or assistant have to apply the slightest bit of effort to answer the phone and press a button. If that is too much for them, the boss should not be in charge of anything and the assistant really isn’t capable of assisting with anything.