Switch on Suspended Delivery in your account. It stops every call, to every number, until you switch it back off. Nobody gets rung about a server you are rebooting on purpose.
Nothing is held while it is on. Mail that arrives during the window is not queued and is not delivered as a call once you switch it off. It is readable in Message History, which is where to look for what came in during the maintenance.
For silencing one phone rather than the whole account, a quiet window on that number does the same thing per phone, and behaves the same way: it drops rather than holds.