Hi,
Yes, what you say seems logical... However, I'm surprised that what I want is not possible. But maybe I was not precise enough

Well, one person has two phones, one cordless DECT and one normal phone on her desk. As this person is often roaming inside the factory, she is called generally on her DECT. But when she is at her desk, she should like to pickup the calls on her desk phone. In other words, I should like to pick up the call on a ringing phone from another phone, even if there is others calls in the same group...
One told me that it was possible with some traditional PABX. I don't know if it's true, however...