Video calling of any kind not functioning? iPhone 4

My girlfriend and I have been trying for literally months now to figure out how to make video calls go through on our iPhone 4s. Before anyone gives the (obvious) answer, yes, we have added each other to our contact lists (on all video apps we've tried). We've mainly been using Skype for voice calls, but when attempting to do video calls we discovered that not only Skype, but every other app we have is dysfunctional. Our Skype apps are both completely up to date and the same versions. We are both connected to WiFi and have access to media features. We've attempted using both front and back cameras. We have tried uninstalling/reinstalling our apps, rebooting iPhones, restoring iPhones, using jailbroken and non-jailbroken OS, pretty much every combination of things we can think of.

We can talk just fine using voice calling features on apps, both iPhone to iPhone and iPhone to PC (when available). Voice call is flawless, works every single time, regardless of the platforms or OS versions or anything.

However, when attempting to use the video call features, we can both see our own camera views on our own screens, but the person we're calling simply gets a solid black screen. There's still voice and sound, just no video display at all. We've tried it between iPhones and iPhone to PC, all we get is a black screen if the iPhone camera is being used.

Video calling did work ONE time, just recently, from my iPhone to her computer (using Skype). She was able to see everything and hear things like it should work. After I hung up and attempted to video call again while she was on her iPhone, though, it was back to the black screen and no video. So, video from iPhone to PC worked once and then never again for no reason apparent to us.

Other video call apps we've tried, like the built-in FaceTime, ooVoo, and some others have also failed to work, showing only a black screen. So, this is apparently an issue with the iPhones themselves, but hopefully someone here would know a working solution.