Some of the phones have a flashing MSG light. When it is pressed it flashes up says Do Not Disturb. What could be the problem?

Identify the button first. Dial feature code 396 then press the flashing button.

In DB Programming look at the key map for the phone(s) in question.

It sounds like a call back msg.
If you dial 368, it will cancel the msg, or if you press it when the other person is at thier desk and they answer the light will go away.
Some offices seem to get a lot of those for some reason.