question about flies

So we've been having a bit of a fly infestation. And we have voip (like Vonage, but a different brand) for our phone. The connection? The little thingamy that connects our internet router to the phone sits on the floor, and it is absolutely covered in flies. Can someone who knows more about entomology than I do tell me what would attract flies to something like this? There isn't anything sticky on it. They are also on the printer and on the router itself. I put some flypaper near the spot where all those are, and it's gotten some but not all of them.