Does voice mail answer when you're on site and dial Intercom 777?
The basics:
Make sure your voice mail ports are in hunt group 7
Make sure the lines to be answered by the Auto Attendant are in CallDistGrp-7
Make sure of what the VMS Hunt Schedule for the line is set to, and if the system is in the necessary mode if it is Day Only or Night Only.
Make sure the lines actually have dial tone, and are the telephone numbers that they should be. Make sure the line are actually ringing into the premises, and not dead or forwarded somewhere else.