try this one,..
<asp:TextBox ID="txtMobileNo" runat="server" MaxLength="10"></asp:TextBox>
<ajaxToolkit:FilteredTextBoxExtender ID="fteMobileNo" runat="server" TargetControlID="txtMobileNo" ValidChars="1234567890">
</ajaxToolkit:FilteredTextBoxExtender>
Accept as answer if solve you problem.