YSK that if you can hear someone over the phone but they can't hear you, then you're the one with a weak signal.

Often people think that the other person would have a weak signal in this scenario. However, this is not the case.

Proof: Suppose that you had a strong signal. If they also had a strong signal, then both of you would be able to hear each other, a contradiction. Similarly, we get a contradiction if they had a weak signal. Thus, we arrive at a contradiction if we assume your signal is strong. Hence, your signal is weak.