getCallingCode() {
		 // If the number is being input in national format and some "default calling code"
		 // has been passed to `AsYouType` constructor, then `this.state.callingCode`
		 // is equal to that "default calling code".
		 //
		 // If the number is being input in national format and no "default calling code"
		 // has been passed to `AsYouType` constructor, then returns `undefined`,
		 // even if a "default country" has been passed to `AsYouType` constructor.
		 //
		if (this.isInternational()) {
			return this.state.callingCode
		}
	}
Returns the "calling code" part of the phone number when it's being input
in an international format.
If no valid calling code has been entered so far, returns `undefined`.
@return {string} [callingCode]