The trick is to sit and stare directly at the person who is making the call.
The first thing they do is drop the volume of the conversation, followed by a turning away of the head. This is often followed by a termination of the conversation.
I have only practiced this on London Buses and various European restaurants up until now and it really works a treat.
If the subject is particularly stubborn, leaning towards him or her whilst still staring often helps. In desperation the final ploy is to ask the person to speak up a bit, as you missed the last part of the conversation.
To date that has never failed !!!
