Put a Call Forward button the Main phone and when they close at night, just hit the Forward button and the call should forward.
We use this at our sites to forward to an answering service, you also need to make sure Redirect is set on the Main phone.
Just wondering, you have a PRI and your not using DID and the ATT is answering every call?
This is a Signature and not part of the answer, it appears on every reply.
This is an Analogy so don't take it personally as some have.
Why change the engine if all you need is to change the spark plugs.