You can use something as below:
<asp:textbox id="TextBox1" runat="server"></asp:textbox>
<asp:regularexpressionvalidator id="rgvPhoneNumber" controltovalidate="TextBox1" runat="server" validationexpression="^[0-9]{9,14}$" errormessage="Please enter a phone number between 9 and 14 digits">
</asp:regularexpressionvalidator>
-Anurag