getNationalDigits() {
		if (this.international) {
			return this.digits.slice(
				(this.IDDPrefix ? this.IDDPrefix.length : 0) +
				(this.callingCode ? this.callingCode.length : 0)
			)
		}
		return this.digits
	}
Returns the part of `this.digits` that corresponds to the national number.
Basically, all digits that have been input by the user, except for the
international prefix and the country calling code part
(if the number is an international one).
@return {string}