TextBox Mask

I have a txtbox for phone that is masked for a phone number. I want to make it so that you have to enter all 10 numbers of the phone.

<asp:TextBox ID="txtphn" runat="server" Width="130px" AutoComplete="off" CssClass="txtInfoBox"></asp:TextBox><asp:MaskedEditExtender runat="server" ID="meephn" Mask="(999) 999-9999" TargetControlID="txtphn"
                    ClearMaskOnLostFocus="False"></asp:MaskedEditExtender>