On a UC320W -- if someone calls in and they dial an extension that isn't valid, but starts with the right digit (in this case '2') then it just beep-beep-beeps and errors out. Sort of an ungraceful way to handle someone who fat fingers an extension.
The "you have reached an invalid extension..." works fine if they hit the 1st digit of a "not defined" plan. It's just if they have 2 as the first digit (it's where the extensions are) that things get ugly. So the client has extensions 201, 202, 203, etc... if someone hits 220 (there's no device or user there) it beep-beep-beeps them and punts them out.