It should be such a simple thing: you're just going to make a video call. If it were a phone call, you'd dial a number and you'd either get through or you'd be directed to the other person's voicemail.
But with video, there are too many variables. Which service will you use? Which software? Is the other person on my buddy list? Are they online? What if you want more than one person on the call? Do all participants have sufficient bandwidth? What if you want to share a screen? And on and on. You can't "just" make a video call with a random person without thinking through these things.
Even with all those questions answered, technology might not cooperate. I used to have a regular video conference with two other people across the country using Skype, but more often than not, the connection was awful. After dropping and reestablishing the call a few times, we'd give up and switch to Google+ Hangouts, which behaved better for us. But in recent months, the opposite has happened repeatedly — Google+ Hangouts first drops video, then audio, and we end up switching to Skype, which has been weirdly reliable (given our history with the service).
As a result, sometimes I can't decide how to contact another person for a video call, even if the person is a Mac user with all the same software and account types I have. If you ever find yourself in the same boat, you may find my ruminations on the matter to be helpful.
Meet the contenders
As I mentioned in Replacing Messages Theater: more screen-sharing alternatives, there are oodles of choices when it comes to videoconferencing. I'm going to discuss just four: Apple's Messages and FaceTime (both of which are built into OS X and iOS), Google+ Hangouts, and Microsoft's Skype. These are among the most popular services, so it's likelier than not that anyone you want to contact by video uses at least one of them. Choosing a service for which the other participants already have an account is usually the path of least resistance.
You may well find a product or service that you and your colleagues find more reliable or easier to use than one of these four. If so, by all means, go with what works best for you — by mutual agreement, well before any particular call is scheduled. The start of a call (or the few minutes before it) is not the right time for someone to set up an account and become acquainted with new software.
Here are the key differentiating features you should be aware of:
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