Backspace & Delete key not working in formatted text (mask editor) control Can someone help?

Dear Sir,

We have created lots of forms throughout application.

In that we have used several controls, one of the control is MASK Editor,

Say for example for telephone and zip code there is 3+3+4 digit and 5+4 in zip code field is text or field width example below.

tel: (???)-???-????
Zip: 00000-9999

but when that field is tabbed by user and move further he cannot come back to this field and do delete of that mis-punched data into that by BACKSPACE or DELETE KEY.

any clue?

Below code is pasted


---------------------------------------------------------
                        <asp:TextBox runat="server" ID="txt_ssn" Width="80"></asp:TextBox>
                        <asp:MaskedEditExtender ID="MaskedEditExtender3" runat="server" TargetControlID="txt_ssn"
                            Mask="999-99-9999" ClearMaskOnLostFocus="false" InputDirection="RightToLeft"
                            AcceptNegative="Left" ErrorTooltipEnabled="True" Enabled="True" AutoComplete="false">
                        </asp:MaskedEditExtender>
---------------------------------------------------------

urgent help will be highly appreciatable.