What you need to do is check where the NITE is pointing too. You probably have an ACD that is pointing to your Meridian Mail. In the Mail it is set up as a VSDN that is set to go to a Time of Day menu. In that Time of Day Menu you will see other menus that the call will go to. One of those is the after hours message. If you have a prompt to leave a message, it is set up in the menu to call answer to the mail box. What you want to do is reset the message on the menu.
Start at the PBX. See what your NITE Number is
Look for it with DNB print out.
If it is an ACDN pointing to your voicemail. Log into your voicemail.
Once logged in press 3 then 4.
You will be at the Voice Admin menu
Press 1 to list the VSDN's. See if your NITE Number is listed there. Once there just follow the menu options to find out where it goes. If you have visio then flowchart it out for future use.
This is vague, but you should be able to get through.