ZTE introduced the foldable Axon M, and Samsung is also working on a similar device called Galaxy X. The latest manufacturer that might enter the race is Oppo, which just patented a similar phone design. It has an outward hinge that, once unfolded, will act as a part of the screen.
The patent entry also suggests there will be a dual camera on the back of the unfolded Oppo. When closed, one of the sides can be used for making calls, while the other can be used for taking selfies with the front camera and what appears to be an LED flash.
The patent renders by Oppo are just for illustration purposes, but we have to mention the massive top and bottom bezels. The side bezels look thin and one of them will be literally non-existent when the phone is folded in its standard form.