This is the 4797th most frequent Russian word.
звонок
"Zвонок" - "bell" or "call."
Here, 'звонок' is used to refer to a phone call received.
Я получил звонок от друга.
I received a call from a friend.
In this sentence, 'звонок' indicates the sound of a ringing bell that marks the beginning of the lesson.
Звонок прозвенел, и урок начался.
The bell rang, and the lesson began.
The word 'звонок' here represents a phone call that is expected to be returned.