Phone line busy is different of user presence busy. Phone line busy is
when your pbx phone line is not free to answer a call, like already busy
in other call, ringing in a call or even when a desk phone device is
off-hook. The user presence busy is a user availability status, that is
not only for when the user is in a call, but can be set manually by the
user in many other situations, like to denote he is busy taking notes,
busy in a presentational meeting or at lunch break.
So, the simple forwarding doesn’t consider the user presence busy status,
but the phone line busy status. User presence is a criteria considered
by rules, as you will see in the next chapter.