diff --git a/lib/phone.rb b/lib/phone.rb
index 978f43f..227cb38 100644
--- a/lib/phone.rb
+++ b/lib/phone.rb
@@ -242,7 +242,7 @@ def format(fmt)
 
     # the default format is "+%c%a%n"
     def to_s
-      format(:default)
+      extension.nil? ? format(:default) : format(:default_with_extension)
     end
 
     # does this number belong to the default country code?
diff --git a/test/phone_test.rb b/test/phone_test.rb
index 8fa3617..e667ff4 100644
--- a/test/phone_test.rb
+++ b/test/phone_test.rb
@@ -101,6 +101,12 @@ def test_to_s
     assert pn.to_s == '+385915125486'
   end
 
+  def test_to_s_with_extension
+    Phoner::Phone.default_country_code = nil
+    pn = Phoner::Phone.new '5125486', '91', '385', '111'
+    assert pn.to_s == '+385915125486x111'
+  end
+
   def test_to_s_without_country_code
     Phoner::Phone.default_country_code = '385'
     pn = Phoner::Phone.new '5125486', '91'