I have found when I have led a meeting and folks are having their own conversation, I just stop talking. If you
stop and wait until they finish, hopefully they will ge the message and wonder why you have stopped. If they say something, just say "I was waiting until you finshed your conversation" and they continue. This is most certainly rude as well as having a cell phone ring. Suggest that folks turn off their phones prior to the start of the meeting.
Sure hope this helps.