The game has no voice over IP (VOIP) so most third party competitive communities use discord to communicate.
Callouts for maps can be found here.
The current map pool for 3rd party community servers and tournaments is Titan, Cloud9, Mart, Airport, Island, and Discovery.
This is a competitive guide. While most people are playing for fun, we find that the enjoyment is when you have a good team and you are all communicating together. We don't expect everyone to know every callout as it comes with experience but please try to know the most common locations and feel free to ask where things are if you didn't understand certain callouts.