My first hack... Back at the days when phones had disks to dial a number. I was a kid of cause, I'm not that old. I used to like to call my grans. Once, when I supposed to go to sleep already, I've found out that there is phone socket in my room (the one connected to the copper wire, that is where the word "phone line" came from).
It took me about a half of an hour to detach handset from the toy phone and about two ours to reverse engineer dialing protocol (you just need to disconnect the line sequentially corresponding number if times).
And after that I've heard my granny's voice. I was literally overwhelmed that it worked.