In the gamemember

Guild › Call to Arms

An officer calls the guild to a world boss, a PvP fight or an event, and everyone is notified right away.

In your game: Guild › Call to Arms

What it does: The officer picks a type (World Boss, World PvP, Defend, Event / Farm or Rally) or a custom template, and sends it. Whoever has the addon gets a popup at the top of the screen, with sound, the place and how many are already on the way, and can reply On my way. A line goes out in guild chat for whoever does not have the addon.
Who sees it: Everyone who has the addon receives it and sees the recent calls. Only officers see Send a call, the templates and Also post to guild chat. Each person chooses in My alerts how they want to be notified.
Where to find it: In the window (J or /gos), Guild tab, Call to Arms sub-tab. Your alerts are right there, in My alerts.

An officer sends at most one call per minute. The wait still applies after reloading the interface.
The popup stays on screen for 30 seconds, one at a time; the newest call takes its place.
A call that arrives less than 10 seconds after the last popup becomes a chat line, with no popup.
With Mute in instances and combat on, inside an instance or in combat the call arrives as a chat line.
A call more than 10 minutes old does not show up.
A long message is cut: 140 characters fit, fewer if it has accents. {zone} becomes the zone the officer is in.
The recent calls list shows the last 10.

After reloading the interface, the recent calls list starts empty. A call is a live alert, it is not stored, and whoever logs in later does not see it.
The guild chat line does not go out if the game is holding chat at that moment, such as during an encounter. The popup goes out anyway.
Two calls from the same officer less than 30 seconds apart: the second is ignored by whoever receives it.