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.
Not sure if this is what you’re describing, but phones like the one you pictured have a reversible tab for use if you mount the phone to a wall - if the tab is sticking up, it keeps the receiver from falling off the cradle.
Here’s a site selling them, but it has a diagram showing where it’s located:
https://www.zcover.com/store/catalog/zCProduct.php?wp=WP_CI881CTR&zPath=212-213-255