U oncall actually talks about this moment when a conference call takes place between 3 or more phone lines.
As soon as two lines speak and raise another line (person) to the phone call (3 people for one call), the third person will receive a push message just before the call enters the phone.
Once the call enters the phone, the third line (the last person attached to the call)
He would receive a message from Foch recording the two telephone numbers already on the line, and actually saying to you, "Pay attention to a conference call."