Search contacts not powerful enough

The search tool works fine when typing a name or email address but does not help when introducing numbers. For instance, let’s say I keep two fields named « client_code » and « VATnr », both are mainly numeric: 45788 and J18468458. If I don’t know email address or name but do have those details, the search tool is not showing anything, so I cannot find the record of that client. I’ve read that it does find phone numbers, but we do not record phone numbers, and it sounds strange that it finds phone numbers but does not find similar number records. Sorry for my English, thanks.