Refer to link below:
JavaScript :Phone Number validation[
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You just have to modify lil bit,and you are done.
Number starts with 97/98:
If you want to check weather number starts from 97 or 98 then just check for first two digits,if not what you expected throw alert.
If user enters any numbers than also it raise error:
<asp:TextBox ID="verification" runat="server" Width="150px"></asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValidator3" runat="server" ErrorMessage=""ControlToValidate="verification" ValidationExpression="^[a-zA-Z]">Enter Characters Only</asp:RegularExpressionValidator>