The Quiet Zone on Chiltern Railways works pretty well during my commute. I've periodically done the not very British thing of politely reminding a someone on the phone that they're seated in a quiet area and apart from one obnoxious individual, it's been fine. I'd be disappointed if Chiltern ever got rid of them...
I've always found a polite wave + smile to attract attention, then pointing at the "No phones" logo works well.
Taking the effort not to interrupt their call is a mark of respect that (hopefully) engenders the same in return.