Only Numbers in Txtbox

 function isNumberKey(evt) {
            var charCode = (evt.which) ? evt.which : event.keyCode
            if (charCode > 31 && (charCode < 48 || charCode > 57))
                return false;
            return true;
        }
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<asp:TextBox ID="txtph2" MaxLength="3"  
      onkeypress="return isNumberKey(event)"
      runat="server"></asp:TextBox>