Calling

Calling in TF2 is communicating with your team to relay information so that everyone knows what's going on. Calling is one of the most important skills that a team can have, it's from calls that organized teamplays are made and it's the backbone of the metagame. It's arguably the biggest difference from Pubs to organized competitive play.

Hence why lobbies are very often frowned upon and disregarded as any demonstration of skill, as the in-game voice system isn't the best solution and there isn't any obligation to use in-game or any type of comms, often resulting in abnormal outcomes from risky plays that wouldn't happen if these plays were called.

Basics

Calls play an essential part in competitive play, so they should be treated as such and given especial attention and work. Calling isn't often trained or practised and most of the calling attributes a player has come from actual competitive experience, so it is natural for new players to feel overwhelmed by it or not being able to make proper calls. A call should be concise and clear, only with relevant information and no gibberish. It should also be spoken in a calm way that is understood by everyone, raging or shouting often only leads to a worse team atmosphere

An example of a good (concise and clear) call would be:

"Scout on choke took 70"

An example of a bad or less clear call would be:

"Scout took a lot, why didn't he die?!?! Please kill him!"

Main Caller

The main caller is the backbone of any team, be it a Highlander or 6v6 team. It is the job of the main caller to communicate with the entire team, informing them when and how pushes, retreats and various other plays are to be carried out. It is essential that the main caller communicates clearly and concisely so actions are carried out as soon as possible. Teams which often fail to listen and react to the main caller generally fall apart due to a lack of focus on their enemies, positioning and the game as a whole.

Having a main caller is not a requirement nor obligation for every team, very often teams use different systems as they see fit to their playstyle, often using no maincaller at all or even 2 callers with strong maincalling presence.

Attributes

A main caller should be a player that has a clear understanding of the metagame and a good gamesense, that knows how his team can take advantage of a certain situation or turn the advantage around with precise and consise calls. Having a strong presence in the team and being respected and heard by your team mates at all times is also necessary. A good microphone is recommended but not obligatory.

Main Calling in 6v6

The main caller is usually the Pocket, the Medic or the Demoman. This is because most 6v6 teams will focus their play around their combo to make use of über and general health advantages. So a class that plays a big part near the combo is often recommended as a maincaller. However this is not true for all cases, as there are teams which have far more experienced players on flank positions that have good main calling abilities.

Main Calling in Highlander

Having a main caller in HL is much more important than on a 6v6 team, a player with a strong presence, that gives clear instructions is a plus, because HL is made up of 9 players in each team, the comms get usually a lot more noisy, often even resulting in a snowballing effect where calls are being talked over by other calls and there is no clear understanding of the direction the team should go. The main caller is usually the Heavy, Medic or Demo as these are the essential combo classes.

Communication system

The in-game voice system is virtually unused in competitive play as it isn't perfect nor offers the amount of audio quality and costumization the players need. So there are plenty of alternatives used, from TeamSpeak to Ventrillo, but the one that has become the staple of the competitive tf2 community is Mumble, as it is very lightwieight, open source, offers good audio quality and customization. Most teams will use this software to communicate and even the most active PUGs or Mixes channels/websites will enforce it, so it is an essential software to the competitive scene.