North American
It used to be common practice to always prompt the caller to provide “a 10 digit phone number” or “phone number, starting with area code”. These days, most callers understand that when prompted for a phone number, they must provide the area code. This additional information may be added to the reprompts just in case the caller has trouble. From a grammar perspective, it's helpful to allow for an optional 1 just in case the caller provides it.

International
Designing for international phone numbers can be tricky. For information on how to dial international numbers, see:

http://www.wikihow.com/Dial-International-Phone-Numbers-from-America

http://countrycode.org/how-to-call