.replace(/^\+[0-9]{1,3}(\s|\-)/) code example
Example: this.value=this.value.replace(/[^0-9]/g,'')
<asp:TextBox ID="txtJobNo" runat="server" class="TextBoxStyle" onkeyup="this.value=this.value.replace(/[^0-9]/g,'')" /><asp:TextBox ID="txtJobNo" runat="server" class="TextBoxStyle" onkeyup="this.value=this.value.replace(/[^0-9]/g,'')" />