Is 「もしもし」 only used when you cannot see the person you're talking with (I once heard that it's somewhat akin to "Anyone there?", which wouldn't make much sense if you could see your conversation partner) or do you also use it when making video calls?
もしもし is like "Hello" and it's used in two kinds of situations:
- As the very first word of the call ("Hello, this is Tanaka speaking.")
- As the word to check if the other person can hear you, when the line is noisy or unstable ("Hello? Hello? Can you hear me?")
In video calls, you can still safely use もしもし in the second situation. In the first sense, and in a video call, I feel もしもし is probably not very common. Many people probably use other nonspecific greetings like おはようございます, どうも. But some people may use もしもし, and it's not that strange.
You definitely do use it on Skype, but only for audio only calls, not when you also have a video feed and can see the other person. It's usually followed by 聞こえますか？ (can you hear me?) when on Skype. Normally on the phone you wouldn't bother asking 聞こえますか？ after you say もしもし because もしもし is already intended to serve that purpose of signalling that you are on the line and are ready to talk but is the other person? But when it comes to Skype you get connection problems and problems with headsets not being configured right, so you tend to also add in 聞こえますか？ to explicitly check if the other person can hear you. If it's a video call and you can see the other person just greet them like you would in person.