Download attached mission and open up in the editor.
There is a crewed artillery piece next to you (vehicle name = a1).
Go into the map. It will show a number of triggers you can trip to initiate fire missions.
Radio Golf - Fires 9 shots from a1
Radio Hotel - Fires 5 shots from a1
Radio India - Fires 3 shots from a1
Radio Juliet - Fires 1 shot from a1
Also there is a trigger to spawn a new artillery piece in front of you. This is scripted to fire at the target 5 seconds after spawn. It's available on Radio Alpha. This is to show it affects spawned in units as well as editor placed ones.
You can make the artillery work normally by commenting out the following lines in init.sqf:
EAST setFriend [WEST, 1]; // <<<< COMMENT HERE
WEST setFriend [EAST, 1]; // <<<< COMMENT HERE
I also checked the independent side relations, but they do not seem to affect the fire missions. I have not checked civilian relations, so it is possible there is an issue there as well.