The voicemail light is fully lit(NOT FLASHING)it's not going away

First off we need to know what color it is.

Green-Dial "0" from the problem phone and that should clear it.

Red-Dial "Feature 99 Feature". If that does not work.
Leave a message in that users mailbox. From that users phone check and delete that message.

If neither of these operations work then you may have a bad port which means you may need a new station card. The part is an ESI(8)-U10-ETU, ESIB(8)-U10-ETU or an ESIE(8)-U10-ETU. The U10 might be different.

Let me know how these options work.