It's both but we are only using the text part of it with custom made Bot system (inside our Discord) to provide maximum focus towards the game. You can run Discord on any Android / iOS phone and you can also run it on a computer if you want. We are barely using voice chat in our group. Main reason is that some of us are not interested in talking and prefer pure game sounds. Since the game is pretty much self explaining, there is no need to constantly follow the chat. Mostly have quick check between missions in order to organize the next step and go on from there.
Since Discord is structured in channels, we are also using the chat to share general information about the game or general talk. We are also supporting GTA and Forza Horizon.
Most of our members are from the UK, US or EU in general. But we also have people from every other part of the world.