Really a guess - an uneducated one at that but I'm guessing it is measuring rf signals transmitting out of the phone more so vs what is being received which is

Really a guess - an uneducated one at that but I'm guessing it is measuring rf signals transmitting out of the phone more so vs what is being received which is why it will also notify if someone is talking on the phone - the phone will be transmitting. There is probably a measurable difference in rf when a phone is trying to send data vs just acting as a receiver, right? From what I can recall, it generally takes more power for a phone to send data to the tower than to receive it. At the same time, you can measure whether Bluetooth is simultaneously being used.