Currently, as of summer 2018, my preferred platform for teaching online lessons is FaceTime. Skype and Google Hangouts also work well, but FaceTime seems to be most reliable, with the fewest connectivity, audio, or visual issues. Most importantly, the audio is consistent and in my experience FaceTime has less delay between the two sides of the call. It's probably only a few hundredths of a second different, but I notice the difference.
Skype's biggest issue is that it will sometimes "mute" one side of the call, when it incorrectly perceives that there is too much background noise. This happens usually when a student is playing a long note. However, here is a way to fix that issue.
Technical Issue: How can I stop Skype audio from cutting out momentarily?
Answer: This problem is usually caused by Skype overcompensating for what it believes to be loud background noise levels. Go into Skype’s Tools menu, then to Options… and select the Audio Settings tab. First, check that the HUE camera is selected as the microphone, and then uncheck the box below marked, “Let Skype adjust my audio settings”. Initially it is best to set the volume slider a little less than halfway along the bar. You can adjust this later to suit your preferences.
Remember to save your settings before closing the window. The sound levels should now remain steady when you’re making calls.
This information was taken from the following resource:
Horn teacher and performer