I just did a quick test on a 5220 - not the same phone but should work the same - I couldn't duplicate the problem.
My next question is this: Does it happen with everyphone or just this one?
If just this one, try the simple thing first and just unplug it and plug it back in. If that doesn't fix it, replace the phone.
If everyphone, I'd like to know what ring type is set to. Ring, ring cont, delay ring, delay ring cont?
Ralph