swap the phone out with another phone, see if it is the phone.
reboot,
next,
delete new messages in MB, the leave one in the box again. Then delete message.
next,
delete MB, recreate MB, leave message.
Check for light after each next item above.
If that does not work, get a piece of black electrical tape. Cut it to fit just over the light, then place tape over the light, lol. Just kidding.