Realistic Artillery Management - FOO Rule

[RERomine]

First, does the FOO rule mean that only FOO's can call in artillery? I'm assuming this is not true, but relates only to FOO units due to their ability to get arty on target faster and more accurately.

Yes, one fire mission can be called by each FOO available. A fire mission can be anywhere from one gun to multiple regiments.

Second, if the above is true then why can't other HQ units call in arty support? I know that it is possible and quite frequently done.

Typically speaking, HQ units calling artillery was just an American thing. It had something to do with the wholesale availability of dry cell batteries and consequently, radios. Most other nations used specialized teams (FOOs - the UK had this down to a science) or in the case with the Russian, artillery was preplanned. As a means to make the rule simple and general, no HQ units can call fire missions, except as mentioned below.

Third, if my first question is true and only FOO's can call in arty strikes - then what happens if they're all killed? Would that mean no more arty 'cept those used in close support?

If all FOO units are destroyed, the A0 unit can call artillery. If no FOO units are purchased, the A0 unit can call artillery.