How do they find out about the inclusion of a recording of a conversation?

Question

Telephony

Tell me, how do scammers find out that the call recording from the phone is turned on?

At the first call, I did not turn on the recording of the telephone conversation when I called and had a sincere conversation.
With repeated calls when turned on, it seemed as if an auto reset was taking place, because the first time the girl did not understand what had happened and called back again.
The third time they didn't call.