If the line is also used for something else e.g. a fax machine, set the Inbound Route -> Pause Before Answer to 10, so the fax answers first.
If it’s just for 911 backup, you could route to Terminate Call -> Play no service message.
Not answering at all seems less desirable; the line is more likely to be in use when needed for 911.
If the possibility of this line actually being used in an emergency is significant, consider routing subsequent inbound calls to the extension that called 911, security guard, front desk, etc.