You can program a Headset Mute button on any blank button; even if you are not using a headset, since the MLX phones share a single jack for headset and handset, the new mute button will mute both.
The MLX phones were lame in several ways: high degree of flakeout on earlier models, no separate headset/handset jacks, Mute button doesn't mute headset, and no backlighting of the display. (All these features are found elsewhere on other Lucent/Avaya phones.)