Wot Tony said: the shoulder launched stuff is only stopped by the Slammers type explosive reacitve armour; the (even more sophisitcated) version used by the TAS; the less sophisticated version used by the likes of the Zaporoskiye (amongst others) or is degraded by plain spaced armour used by, for example, the Hiroseki army (the latter also works against ATGWs as well).
There is an optional rule for allowing the Slammers style reactive armour to work against ATGWs as well (though rather less effectively: Using ADS against missile attacks, Supplement 4).
Calliopes – along with some other rapid fire weapons like tribarrels – have (if they have the AI to control it) the ability to shoot down incoming ATGWs ained at the vehicle in question or – optionally and at a reduction in success – when aimed at other vehicles on the same ‘net’ (ie other vehicles in the detachmet or even the force if the scenario permits it: the Slammers often arrange to provide AA cover for other units). Page 122-124 main rules, Anti-Artillery and anti-ATGW capability, Supplement 4 and the sidebar on the same page.
The same systems (either red triangles or blue triangles if they aren’t as good marked on the play sheets) will also attack incoming artillery rounds from off table or even mortar rounds if fired off table (optional rules: Steel Rain, Supplement 1). On table, the mortars get through and hit the roof!