I have noticed phones with a handset (like the one in the image) have a little cover that resembles something like a cold camera shoe under the bottom of the handset’s top speaker holder. Is there a use for it? It has a line bump in the middle, but it doesn’t go all the way from both sides, it leaves a gap. I have also seem some of them have extra space on the top of the cover, and some don’t.
An old hack that not many knew was that you could tap the hang up switch to dial numbers. Example, tap 3 times = 3, tap 10 times = 0.
Rotary phones did the same thing which is why you had to wait for the whole thing to spin back, it was tapping the line to dial out.
Trainer at my Judo club used that to call my parents with the gym-phone that had its disc locked with a padlock, when I got injured. He was great.