If you have a main line number set up on a hunt group configuration, where the line uses a 'follow me' rule to call other extension lines then we will need to remove the follow me ruling for you for the voicemail message to be active.
For example if the follow me rule on a main line number is routed to firstly call reception and then if no answer it rings another line and so on, then you will need to contact our Support Team so that we can remove the follow me ruling to enable the voicemail message on your main line to be active.