Busy box is not an operating system. Busy Box is a shell. You seem to be unsure of the differences between the two, so if I may offer a car-like comparison, A shell is like a steering wheel in a car. It has no relevance to what is under the hood, how many wheels the car has, or even if it's a car at all!
However, to expand on that, busybox is more like a motorcycles handlebars, than a cars steering wheel. It's possible to put motorbike handlebars in a car, but it would be pretty silly, and awkward.
So, we know that because you're using Busybox, this is a motorbike. This motorbike doesn't have the required number of wheels, nor the trunk space, or the passenger capacity required for FreePBX.
Makes sense now?