ACARS
Isn't that ON VOICE indicator on the ACP of the radio that is normally used for ACARS - the set is then working for datalink. Under pilots discretion it is allowed to switch that set from ACARS use to voice use, for instance if you fly near Cyprus where you are supposed to maintain simultaneous contact with Nicosia and Ercan radar and you might still want to monitor GUARD (121.5)
In order that you do not forget to switch the set back to ACARS use, your attention is drawn to the ON VOICE status by a light.