RequiredFieldValidator is not working for empty textbox.

Thanks, I checked AutoPostBack. It is "False".
<asp:TextBox runat="server" ID="phone" MaxLength="10" Columns="10" Width="90px" CausesValidation="True" /> <asp:MaskedEditExtender ID="phone_MaskedEditExtender" runat="server" CultureAMPMPlaceholder="" CultureCurrencySymbolPlaceholder="" CultureDateFormat="" CultureDatePlaceholder="" CultureDecimalPlaceholder="" CultureThousandsPlaceholder="" CultureTimePlaceholder="" Enabled="True" TargetControlID="phone" Mask="(999)-999-9999" ClearMaskOnLostFocus="False"> </asp:MaskedEditExtender> <asp:RequiredFieldValidator runat="server" ID="RequiredFieldValidatorPhone" ControlToValidate="Phone" ErrorMessage="Phone is required." />
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