I created a button on a twelve button set with a LED. I used F11 11 21. The calls ring to Extension 21 but voice mail does not activate. Why is that? I want to be able to have the user at Extension 11 move to Extension 21 at night and have the user either pick up the calls at his night extension 21 or be able to have callers leave a voice mail message at Extension 21, his night time extension.