There is an entire wiki page for this which explains several different ways of doing it: http://community.bistudio.com/wiki/Artillery_Module
Also there's a script that does this without any Modules. I couldn't find a link to download so I uploaded it myself, it was created by JimSolo and he released it in this thread: http://forums.bistudio.com/showthread.php?t=109519&page=2
download link: http://www.megaupload.com/?d=CQJ3BNJ0
To use it:
- download FireSupport.sqf and place it in your mission folder
- create a marker where you would like artillery support, and name it "target1".
- create a trigger and set its activation to Radio Alpha, and set its text to "Call Fire Support"
In the 'On Act.' field for the trigger, insert the following code:
_anyname = ["target1","G_Camel_HE",24] execVM "FireSupport.sqf";
- If you want to change how many rounds are fired, change "24" to the desired number
And that's it :)
To test it out, go in game, get a good view of where you placed the marker, and press 0, 0, 1, to call it from the radio.