validation of a text box

I have a mobile textbox
 <asp:TextBox runat="server" " ID="MobileNO"></asp:TextBox>
and I have 4 button's like this
  <asp:Button ID="Button1" runat="server" Text="Save" />  <asp:Button ID="Button2" runat="server" Text="Save" />  <asp:Button ID="Button3" runat="server" Text="Save" />
  <asp:Button ID="Button4" runat="server" Text="Save" />
, which perform some upgrades in db. but I have to ensure that this mobileno is filled before submitting with valid numeric, length  of 10 and it is compulsory..
how can i achieve that and I cant make   one validation group, because everybutton has someother validations apart from mobile