Eli5: Why does the coin drop after hanging up the pay phone

Question

I always see in the movies that when they hang up a pay phone, you hear a coin drop. What’s the reason for this?

In: 244

Answer 1

When you would fund the pay phone with the appropriate coinage, there was a sort of escrow relay that would hold the coin until it was verified you had connected and completed a call.
If the line connected and you made a call, then when the phone hung back on the receiver, it would signify the end of the call and the coin would drop into the coin box.
If the call wasn’t completed, it would return the coin to the refund chute.