Varies with context. Local pub session in bar, conversation goes on round it. Can go quiet for a singer - or not.

In a club, turning round and raising and eyebrow seems to work well. Have known the wife or partner of a paid act chat during a floor spot preceding - there's living dangerously.

Performers need to cope with it. Very good guitarist of may acquaintance came to a dramatic halt when someone coughed or spoke during his performance and rather flouncily said 'Do you mind, I [heavily emphasised] am playing'. Not a good idea to suffer sense of humour failure. Yes, he's very good, but the point is to entertain and inform the audience, not to let them know about any excess self-esteem.

Having someone go on talking when you're trying halting to sing is rough. Also very effective feed-back.

If someone is being a total tw*t, see what allies you have ine hte room.

Presently I find a small club audience will listen, probably to see if I can get through the piece without it falling over....

As for mobile phones...