function generateNationalNumberDigits(phoneNumber) {
	return phoneNumber.formatNational().replace(/\D/g, '')
}
Generates national number digits for a parsed phone.
May prepend national prefix.
The phone number must be a complete and valid phone number.
@param  {object} phoneNumber - An instance of `PhoneNumber` class.
@param  {object} metadata - `libphonenumber-js` metadata
@return {string}
@example
getNationalNumberDigits({ country: 'RU', phone: '8005553535' })
// returns '88005553535'