When users choose to participate in a meeting using the web or Lifesize app and then also choose to dial in via phone for audio, they often forget to mute their computer mic and speakers which causes an echo loop. These users are also counted twice in the participant list.
Other meet me type services use an onscreen PIN to associate the phone call to the web or client app session. An on screen message requests "Dialed in via phone? Enter this PIN...." Once associated, the application audio devices are muted, the echo is killed and the person is only counted once.