When I've done this before, I've had calls during the day point a a hunt group. Set the night destination of that group to a dummy user you create. Set the dummy users twinning destination to be the answering service. Then, create a button on the receptionist's phone to put that group in night mode.

There are likely a few other ways to do this too. That's what I like about IPO--20 ways to do the same thing.