Optimize Validation Code by moving duplicate code structure into chec…

…kShortCodeOverlapping method and introduce ShortCodeUseable enum, to controll the usage of the isUsable... functions of the numberplan object.

src/test/groovy/de/telekom/phonenumbernormalizer/PhoneNumberValidatorImplTest.groovy

@@ -529,7 +529,7 @@ class PhoneNumberValidatorImplTest extends Specification {
         "0175 1155555"              | "DE"       | PhoneNumberValidationResult.IS_POSSIBLE_NATIONAL_ONLY                // number starts with NAC, mandatory mobile NDC follows, so subscriber number is not overlapping with short codes - but SN length for this NDC is 7
         "0175 11555555"             | "DE"       | PhoneNumberValidationResult.TOO_LONG                                 // number starts with NAC, mandatory mobile NDC follows, so subscriber number is not overlapping with short codes - but SN length for this NDC is 7
         "0175 115555555"            | "DE"       | PhoneNumberValidationResult.TOO_LONG                                 // number starts with NAC, mandatory mobile NDC follows, so subscriber number is not overlapping with short codes - but SN length for this NDC is 7
-        "0203 115"                  | "DE"       | PhoneNumberValidationResult.IS_POSSIBLE                              // number starts with NAC, optional fixed line NDC follows, SN equals short code and the local service is targeted regardless of caller location.
+        "0203 115"                  | "DE"       | PhoneNumberValidationResult.IS_POSSIBLE_NATIONAL_ONLY                // number starts with NAC, optional fixed line NDC follows, SN equals short code and the local service is targeted regardless of caller location.
         "0203 115555"               | "DE"       | PhoneNumberValidationResult.INVALID_PREFIX_OF_SUBSCRIBER_NUMBER      // number starts with NAC, optional fixed line NDC follows, SN starts with short code (overlapping) => assuming NDC is intended, which means SN is wrong
         // using IDP+CC within the region
         "+49115"                    | "DE"       | PhoneNumberValidationResult.INVALID_COUNTRY_CODE                     // number starts with IDP+CC, normally NDC would follow, but that equals short code => inside germany explicitly not allowed (see BnetzA)