@Maltimore seems it was added as a public beta in February 2017:

https://signal.org/blog/signal-video-calls-beta/

Then made public for all in March 2017:

https://signal.org/blog/signal-video-calls/

I've been using Signal since December 2017, and somehow I never found how to make video calls, and as such assumed it just wasn't possible.

I've been recommending Wire for video calls instead, apparently for nothing. 😌