the user has one telephone of 2 lines who brings the conference button and the button of hold to be able to speak to the same time the line 1, line 2 and the user, then appears noise and the three cannot speak, this single one happens when the lines are of 24v and 48v but when the lines are of 24v is no problem nor when the lines are of 48v in this cases can speak all three and there isn´t noise.
regards
aprieto