system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do systems on the other side of a phone call know what buttons you're pressing.

For example: I just called a phone number to activate a visa gift card, and the system told me to enter the number of the card on my dial pad. How does that system know what buttons I'm pressing?
gpt
Your phone beeps when you press the buttons - the same beeps that tell the phone company which number you're dialling on a landline. The system on the other end listens for the beeps and knows which button you pressed. Each button actually makes two tones together - one for each column and one for each row.