How to enter Phone in this format XXX-XXX-XXXX

Register AjaxControlToolKit like

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="uc" %>

and design script

<asp:TextBox ID="txtPhone" runat="server"></asp:TextBox>
           <uc:MaskedEditExtender ID="msktxtPhone" runat="server" TargetControlID="txtPhone"
                                                Mask="999-999-9999" MaskType="None" ClearMaskOnLostFocus="false"  />