PhoneNumber.java

public class PhoneNumber {
    private final String areaCode;
    private final String phoneNumber;

    private PhoneNumber(String areaCode, String phoneNumber) {
        this.areaCode = areaCode;
        this.phoneNumber = phoneNumber;
    }

    public static PhoneNumber create(String areaCode, String phoneNumber) {
        String sanitizedAreaCode =
            (areaCode == null ? "" : areaCode)
                .trim()
                .replaceAll("[^a-z+]", "")
                .replaceFirst("^\\+?49", "")
                .replaceFirst("^([^0])", "0$1");

        String sanitizedPhoneNumber =
            (phoneNumber == null ? "" : phoneNumber)
                .trim()
                .replaceAll("[^a-z+]", "");

        // TODO handle same/similar areaCode/phoneNumber
        return new PhoneNumber(sanitizedAreaCode, sanitizedPhoneNumber);
    }

    public String join(String separator) {
        return areaCode + separator + phoneNumber;
    }

    public String phoneLink() {
        return "tel:" + (areaCode + phoneNumber).replaceFirst("^0", "+49");
    }
}