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Kind of difficult to suggest the best solution without knowing your setup.
Here are the KNOWNS
You have a receptionist (probably a Standard user)
You have an IVR menu
You want the receptionist to divert calls to a Message-Only-Ext. for out of hours.

Here are the UNKNOWNS
How does a caller reach the receptionist (Through an IVR menu? Through a Call queue without an IVR menu? directly? do they hear any greeting before connecting - if yes you probably don't want to play that greeting before diverting to out of hours)

Whatever the case, I'd expect you want your receptionist to either (1) use the "Forward all calls" or a "Custom Rule" to divert her calls...Or (2) give her Phone System admin access.