diff --git a/activesupport/test/core_ext/numeric_ext_test.rb b/activesupport/test/core_ext/numeric_ext_test.rb
index 8636da1b93..42109a8f17 100644
--- a/activesupport/test/core_ext/numeric_ext_test.rb
+++ b/activesupport/test/core_ext/numeric_ext_test.rb
@@ -400,6 +400,12 @@ class NumericExtFormattingTest < ActiveSupport::TestCase
     assert_equal '1 Million', BigDecimal("1000010").to_s(:human)
   end
 
+  def test_to_formatted_s_is_deprecated
+    assert_deprecated do
+      5551234.to_formatted_s(:phone)
+    end
+  end
+
   def test_in_milliseconds
     assert_equal 10_000, 10.seconds.in_milliseconds
   end