system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a phone on speaker transmit your voice but not the other end of the conversation.
gpt
You can think of the phone having two lines, one that's coming in carrying the callers voice, and one that's going out carrying your voice.

When the callers voice is coming out of the speakers the microphone listening for you voice also hears it.  But the phone knows that it should take sound that it's putting out from the callers voice and subtract it from the noise it's receiving from your voice.