digitsWithoutCountryCode(text) {
    let digits = (text || '').replace(/[^\d]/g, '');
    const extraDigits = digits.length - 10;
    if (extraDigits > 0) {
      digits = digits.substr(extraDigits);
    }
    return digits;
  }
Gives back just the phone number digits as a string without the
country code. Future-proofing internationalization where the country code
isn't just +1.

@param {string} text
@private