Thank you, i don't clearly understand the response.
When the Phone⇔Line 1/2 is engage in the setup/options/phones, the monitor 1/2 do not mute any more when i insert the phone plug in 3/4.
If i put the setup/options/phones/Phone⇔Line on OFF the mute is done when i plug the phone in 3/4. Is it normal ?

But the naming of the feature is a bit cryptic, to me as well.
I always have to look up the corresponding section in the user's manual, as "Phones <=> Line" isn't any intuitive because I would never associate the "<=>" with a toggling function.
Might be better called "Enable Manual Output Select"?
This would indicate its two functions: a) That it *allows* a selection of  which output(s) are active and b) that this selection is *manual* (via VolKnob) and overrides the sensor-based muting of line-out (if enabled by the Mute vs. TRS1/2 or TRS3/4, which is also wrongly named btw as it actually means PH1/2 or PH3/4. TRS1/2 is the rear unbalanced line out).