That may the way the phone system says "on another call" if the software is French, or has some French/ Canadian bug. You will also notice it only happens for internal callers, as opposed to outside callers. If this is true it happens by design. But, I am not familiar with French software, so I cannot say for sure.
What do you want the phone to do while he is on another call?
If you want it ring, remove any DND on busy setting, and any fwd on busy setting, and confirm it has more than one IC.
Also, check what happens if it is an outside caller.
Check the fwd on busy setting (it is best to leave this to none no matter what your config is)
It may be forwarding on busy to an invalid DN. (this is my best guess)