getNumberValue() {
		const {
			digits,
			callingCode,
			country,
			nationalSignificantNumber
		} = this.state

	 	// Will return `undefined` if no digits have been input.
		if (!digits) {
			return
		}

		if (this.isInternational()) {
			if (callingCode) {
				return '+' + callingCode + nationalSignificantNumber
			} else {
				return '+' + digits
			}
		} else {
			if (country || callingCode) {
				const callingCode_ = country ? this.metadata.countryCallingCode() : callingCode
				return '+' + callingCode_ + nationalSignificantNumber
			}
		}
	}
Returns a E.164 phone number value for the user's input.

For example, for country `"US"` and input `"(222) 333-4444"`
it will return `"+12223334444"`.

For international phone number input, it will also auto-correct
some minor errors such as using a national prefix when writing
an international phone number. For example, if the user inputs
`"+44 0 7400 000000"` then it will return an auto-corrected
`"+447400000000"` phone number value.

Will return `undefined` if no digits have been input,
or when inputting a phone number in national format and no
default country or default "country calling code" have been set.

@return {string} [value]