What is the thing that resembles a camera shoe under the handset holder found on telephones with a handset used for?

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.