Again, it will all depend on how incoming calls are routed.
Some calls to a queue, some DID's to extensions, some to a receptionist, etc. You can define "Office Hours", and each "destination" has a way to deal with call during and after office hours, if you choose to use it. There isn't one, single option that deals with all incoming calls unless they are all on the same trunk group, going to a single destination.
A dummy extension can be set up and any voicemail left on it can be accessed by anyone. something such as that could be used for after hour calls. MWI on a mailbox such as this will not notify multiple sets unless they have a secondary key with the mailbox extension number.