Free calling has been a part of Messenger for quite some time, but it saw a quiet refresh with the launch of Messenger 3 last year. Since then, we've decided to make it even easier to call people on Messenger. Look for the call button at the top right of your conversations in the coming weeks!
I've loved using it to have crystal clear conversations with my friends, my mom, and my dad over these past months. I hope it works well for you, too :-)