Add SharesNdcWith

src/PhoneNumbers/PhoneNumber.cs

@@ -216,4 +216,12 @@ public override string ToString() =>
     /// <returns>The string representation of the value of this instance as specified by the format.</returns>
     public string ToString(string format) =>
         CountryInfo.GetFormatter(format).Format(this);
+
+    /// <summary>
+    /// Gets a value indicting whether this <see cref="PhoneNumber"/> shares a national destination code with the specified <see cref="PhoneNumber"/>.
+    /// </summary>
+    /// <param name="phoneNumber">The <see cref="PhoneNumber"/> to compare against.</param>
+    /// <returns>True if the both phone numbers share a national destination code, otherwise false.</returns>
+    internal bool SharesNdcWith(PhoneNumber phoneNumber) =>
+        NationalDestinationCode?.Equals(phoneNumber.NationalDestinationCode, StringComparison.Ordinal) == true;
 }

test/PhoneNumbers.Tests/PhoneNumberTests.cs

@@ -166,6 +166,29 @@ public void Parse_Value_With_Custom_ParseOptions()
         Assert.Equal("The value '+441142726444' could not be successfully parsed into a phone number for any country enabled in ParseOptions.", exception.Message);
     }
 
+    [Fact]
+    public void SharesNdcWith()
+    {
+        var phoneNumber1 = TestHelper.CreateGeographicPhoneNumber(
+            trunkPrefix: default,
+            ndc: "12345",
+            sn: "667788");
+
+        var phoneNumber2 = TestHelper.CreateGeographicPhoneNumber(
+            trunkPrefix: default,
+            ndc: "12345",
+            sn: "667788");
+
+        Assert.True(phoneNumber1.SharesNdcWith(phoneNumber2));
+
+        var phoneNumber3 = TestHelper.CreateGeographicPhoneNumber(
+            trunkPrefix: default,
+            ndc: "12346",
+            sn: "667788");
+
+        Assert.False(phoneNumber1.SharesNdcWith(phoneNumber3));
+    }
+
     [Fact]
     public void ToString_Returns_Default_Formatted_PhoneNumber()
     {