key down e.suppress key

i have a telephone number textbox that i have used the key down event to suppress any other keys besides 0-9 and the backspace key. i have a label beside it.
i want to display a message on lbltelephone when the user presses an alphabetic key that the textbox is for numbers only,and the message should disapper when the user inputs correct values...
currently i have

e.suppresskeypress=true
lbltelephone.text="no"

but the message does not disappear when user enters correct values and even if focus is lost