I agree with you, it could be useful to configure it like you said. But in this case, you have to pay lines for each extension you have to add. I mean if the number given to the trunk SIP is +331234500 and I want to have 2 extensions (01 and 02 for example), I will have to pay to have two more lines to have +331234501 and +331234502. Am I right?
However, we are using a different working in my company. Let me explain how we work:
When someone call us, there is an audio message telling to choose between 1 or 2 to get the commercial office or the technical office. If someone choose 1, just the phone at the commercial office will ring.
But if someone choose 2, the phones located in the two offices will ring. And that's the part I really not understand where to configure it.
Tell me in you need more informations or if something is not clear about what I said.
Cheers,
David