diff --git a/csharp/PhoneNumbers.Test/TestMedataManager.cs b/csharp/PhoneNumbers.Test/TestMedataManager.cs
index 50c6c4a1..18234dc1 100644
--- a/csharp/PhoneNumbers.Test/TestMedataManager.cs
+++ b/csharp/PhoneNumbers.Test/TestMedataManager.cs
@@ -60,6 +60,11 @@ public void TestAlternateFormatsFailsGracefully()
         /// </summary>
         // IMetadataLoader/EmbeddedResourceMetadataLoader/SetMetadataLoader are Obsolete for external
         // callers only; this test exercises them directly as this project's own internal API.
+        // The Obsolete message names no replacement — the member is slated to become internal, not
+        // to be superseded — so there is nothing for these calls to migrate to, and covering the
+        // deprecated entry point is exactly what the two tests below are for. CodeQL's
+        // cs/call-to-obsolete-method still reports them; dismiss those alerts rather than
+        // rewriting the tests away from the API under test.
 #pragma warning disable CS0618
         [Fact]
         public void SetMetadataLoader_RoutesLookupsThroughCustomLoader()
diff --git a/csharp/PhoneNumbers.Test/TestPhonenumber.cs b/csharp/PhoneNumbers.Test/TestPhonenumber.cs
index efd29e30..5798345f 100644
--- a/csharp/PhoneNumbers.Test/TestPhonenumber.cs
+++ b/csharp/PhoneNumbers.Test/TestPhonenumber.cs
@@ -59,6 +59,23 @@ public void TestNonEqualWithItalianLeadingZeroSetToTrue()
             Assert.False(numberA.GetHashCode() == numberB.GetHashCode());
         }
 
+        [Fact]
+        public void TestItalianLeadingZeroSetterWritesNumberOfLeadingZeros()
+        {
+            // Exercises the obsolete ItalianLeadingZero property setter itself, not
+            // SetNumberOfLeadingZeros directly, since that setter is what delegates to
+            // SetNumberOfLeadingZeros(value ? 1 : 0) under the hood.
+#pragma warning disable CS0618 // intentionally exercising the obsolete setter
+            var builder = new PhoneNumber.Builder()
+                .SetCountryCode(1).SetNationalNumber(6502530000L);
+            builder.ItalianLeadingZero = true;
+            Assert.Equal(1, builder.NumberOfLeadingZeros);
+
+            builder.ItalianLeadingZero = false;
+            Assert.Equal(0, builder.NumberOfLeadingZeros);
+#pragma warning restore CS0618
+        }
+
         [Fact]
         public void TestNonEqualWithDifferingRawInput()
         {
diff --git a/csharp/PhoneNumbers/Phonenumber.cs b/csharp/PhoneNumbers/Phonenumber.cs
index df6b261b..9aee2de0 100644
--- a/csharp/PhoneNumbers/Phonenumber.cs
+++ b/csharp/PhoneNumbers/Phonenumber.cs
@@ -167,7 +167,10 @@ public string Extension
             public bool ItalianLeadingZero
             {
                 get => MessageBeingBuilt.ItalianLeadingZero;
-                set => SetItalianLeadingZero(value);
+                // Goes straight to the replacement rather than through the equally-obsolete
+                // SetItalianLeadingZero: that method is nothing but
+                // SetNumberOfLeadingZeros(value ? 1 : 0), so this is the same write.
+                set => SetNumberOfLeadingZeros(value ? 1 : 0);
             }
 
             public bool HasNumberOfLeadingZeros => MessageBeingBuilt.HasNumberOfLeadingZeros;