I think the OP has two questions, one of which needs some clarification.
Whatever number you use for your restaurant, it doesn't matter who hosts it as to how your customers call it.
Whether they have to dial the area code first or not DEPENDS on the area THEY LIVE IN and where the number YOU USE FOR YOUR RESTAURANT is, coupled with the rules for their phone service.
If I live in town B, which borders town A, I dial 7 digits, no area code, to call town A.
If I move to town C, which is 15 minutes away, the phone company in that area requires that I dial the area code to call town A.
Of course people with cell phones dial the complete number regardless.