What you need is one text box with input validation for mobile number, e.g.:
<asp:TextBox ID="txtMobileNo" runat="server" />
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server"
ControlToValidate="txtMobileNo" ErrorMessage="Please enter a 10-digit mobile number" ValidationExpression="\d{10}" />
Read more:
RegularExpressionValidator[
^]