inputDigits(nextDigits, state) {
		const { digits } = state
		const hasReceivedThreeLeadingDigits = digits.length < 3 && digits.length + nextDigits.length >= 3

		// Append phone number digits.
		state.appendDigits(nextDigits)

		// Attempt to extract IDD prefix:
		// Some users input their phone number in international format,
		// but in an "out-of-country" dialing format instead of using the leading `+`.
		// https://github.com/catamphetamine/libphonenumber-js/issues/185
		// Detect such numbers as soon as there're at least 3 digits.
		// Google's library attempts to extract IDD prefix at 3 digits,
		// so this library just copies that behavior.
		// I guess that's because the most commot IDD prefixes are
		// `00` (Europe) and `011` (US).
		// There exist really long IDD prefixes too:
		// for example, in Australia the default IDD prefix is `0011`,
		// and it could even be as long as `14880011`.
		// An IDD prefix is extracted here, and then every time when
		// there's a new digit and the number couldn't be formatted.
		if (hasReceivedThreeLeadingDigits) {
			this.extractIddPrefix(state)
		}

		if (this.isWaitingForCountryCallingCode(state)) {
			if (!this.extractCountryCallingCode(state)) {
				return
			}
		} else {
			state.appendNationalSignificantNumberDigits(nextDigits)
		}

		// If a phone number is being input in international format,
		// then it's not valid for it to have a national prefix.
		// Still, some people incorrectly input such numbers with a national prefix.
		// In such cases, only attempt to strip a national prefix if the number becomes too long.
		// (but that is done later, not here)
		if (!state.international) {
			if (!this.hasExtractedNationalSignificantNumber) {
				this.extractNationalSignificantNumber(
					state.getNationalDigits(),
					(stateUpdate) => state.update(stateUpdate)
				)
			}
		}
	}
Inputs "next" phone number digits.
@param  {string} digits
@return {string} [formattedNumber] Formatted national phone number (if it can be formatted at this stage). Returning `undefined` means "don't format the national phone number at this stage".