In the broadest sense, Calling refers to using the team chat to broadcast information about your character to the other members of your party. This is easily achieved by holding down the left Control key while clicking on an area of the screen. For example, control-clicking on an enemy (in addition to causing you to attack it) will generate a text message in the team chat that says "I am attacking [enemy name]!" This particular example is also named a target call. (Control-shift-clicking will instead announce "I am targeting [enemy name]!" and will not attack the target. Both methods will cause henchmen on your team to focus on the called target.)
Party members can automatically select a called target by hitting the "T" key. All their attacks and spells will be directed to this target.
This is what most people refer to when they say "Call!" or "Who's calling?" It allows a team to focus on a particular target, without the difficulties of typing while fighting. This is essential in PvP, and in harder missions and quests, since if the team does not focus well on particular targets enemies may present a significant challenge.
Stategically speaking, having a party of 6 (or more) players spamming the team chat with what their character is doing is extremely confusing, especially in the heat of battle. Therefore, organized parties will generally designate one player as having "the call", who is responsible for identifying which target the party should focus their attention on. Warriors are traditionally chosen as callers because they are closest to the enemies and have the best view of enemy units. Other classes tend to be in the back and may click on the wrong target in a group of enemies moving in the heat of battle. Still, the Caller should be a player with experience in fighting the specific enemies of the area the party is in and in the general tactics of the game.
Calling covers more than just attacking, however. Other cases for calling include:
- Notifying your party you have a negative condition or hex on you. Monks, Mesmers and Necromancers have spells that can alleviate such effects or at least take advantage of them.
- Notifying other party members of special spells (or skills in general) you are casting. For example, a Ranger using Healing Spring may wish to announce that for other members to rally around him/her. The most common example of this is monks coordinating their healing and protection spells.
- Indicating which player is resurrecting a team mate to prevent the whole team from ceasing their current action.
- Warning party members about an impending enemy action. By control-clicking on the Skill Monitor under an enemy's name players can warn their teammates about the actions of that enemy. For example, to warn party members about a Mahgo Hydra's incoming Meteor Shower.
- Calling out one's skill bar in town to co-ordinate builds.
- Warning your party members your Health and Energy numbers