hi sanket2797,
You can use this wich proper validation.
<asp:TextBox ID="txtOfficerMobile" CssClass="form-control" MaxLength="10" TabIndex="3" runat="server"></asp:TextBox>
<small style="color:red">Example:9998887771</small>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" ValidationGroup="Sub" SetFocusOnError="true" ControlToValidate="txtOfficerMobile" runat="server" Display="Dynamic" ForeColor="Red" ErrorMessage="मोबाइल न0 दर्ज करे |"></asp:RequiredFieldValidator>
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" ValidationGroup="Sub" runat="server" SetFocusOnError="true" Display="Dynamic" ControlToValidate="txtOfficerMobile" ForeColor="Red" ValidationExpression="^[7-9][0-9](\s){0,1}(\-){0,1}(\s){0,1}[0-9]{1}[0-9]{7}$" ErrorMessage="कृपया मान्य मोबाइल न0 दर्ज करे |"></asp:RegularExpressionValidator>
<cc1:FilteredTextBoxExtender ID="FilteredTextBoxExtender1" runat="server" TargetControlID="txtOfficerMobile" ValidChars="1234567890" FilterMode="ValidChars"></cc1:FilteredTextBoxExtender>