That's a great idea, but I'd be worried about people forgetting to call later after deciding not to call while the phone line is "closed".

If you were really enterprising you could put up a form during off hours that let them specify when they'd like to be called back during on hours.

or have voice mail.

Or voicemail. But if you want to really empower your users, let them pick a time to be contacted and then call them then. Voicemail is leave it and forget it and when I call a company and leave a voicemail it feels like a black hole that my message may never escape. Maybe that's just me.