According to one word per minute, in this way, in the first minute, the teacher informs the students 1, and in the second minute, the teacher and student A inform the students 2 and 3 respectively; In the second minute, the teacher 1, 2 and 3 notified 4, 5, 6 and 7 respectively, and in the fourth minute, the teacher 1, 2, 3, 4, 5, 6 and 7 notified 8, 9, 10 and1/respectively. Teacher in the fourth minute, 1, 2, 3,. . . . Notify 17, 18, 19, 20, 2 1, 22, 23, 24, 25, 26, 27, 28, 29, 30, 3 1 respectively. Of course, this only considers the shortest time. In practice, we should also consider whether the phone can be connected and whether everyone knows the other party's phone number.