In the invitation ask people to join from somewhere quiet

Ask people to mute their phones when not talking. If their phone does not have a mute button, they can mute/unmute their lines by pressing  6

My participants are a rabble and don’t mute their phones!

You can forcibly mute all lines by pressing  5

There will now be no background noise.

Your participants cannot unmute themselves.

If you would like the participants to be able to unmute themselves press  4

They can now unmute themselves by pressing  6