I ve added an extra piece of code along with the call artillery func in the trigger:
som1 setVariable [""pool"", [0]];
The pool here seems to relate with random activation of SCOM missions.
The complete code for the trigger is:
[[""artillery_barrage""], player, [[barrage1, [1, 7, 9]]]] call BIS_SOM_addSupportRequestFunc; som1 setVariable [""pool"", [0]];
Replace Barrage1 (name of my barrage) and som1 (name of my own SOM)with your own namings.