I keep the phone to my ear, stay silent, and hear: HHHHEEELLLLOOOO!!!!! HHHHEELLLOOOOO???? Hello! Hello! HEEELLLLLLLLO? HELLOOOOOOOO!!! Over and over again. In my own voice. Some mocking, some shouting, some sounding deeper, and some sounding higher. But all are in my voice.
The phone calls where you hear your own voice are usually failed international calls. There is supposed to be an echo canceler that prevents you from hearing this but sometimes they fail and you hear yourself a second or two later. When they started using digital devices for echo cancellation they failed in more interesting ways. You would sometimes hear scrambled, Mickey Mouse, and chopped up bits of what you just said.
Back in the 70's when a friend and I were bored nerds, we would sometimes record CB radio personalities who we didn't like, mess with the recording (adding clips from Cheech and Chong records was our favorite), and transmit them back on the channel days later. A typical example would be:
"Breaker 1-9. This here's Lonedog I was wonderin' if y'all cord drup mommy pifflespiddle grooooovy man crutch blerb doop fer me. 10-4?"
We'd laugh like idiots as there would be a minute of:
"Comeback at us, Lonedog? Whatcha sayin'?"
"Lonedog, y'all right there?"
After a few weeks of this, they finally figured out what we were doing so it wasn't fun any more.
This was a little work with tape. Pranks like this are lot easier these digital days.