The Counter-Arty Strike costs 2 TP and can only be used when you are under arty attack. If the attack ends before the call is completed, the call gets canceled and you will not get your points back.
If, however, the counterattack works, the source is triangulated and attacked by the Allies and forces the enemy to abort the next one, whenever that may be – even if it is an arty blitz! However, the Counter Arty will not cancel the current strike.
Mind that, since your time frame is so small, you might want to build an antenna to help your signaler call your counter-arty strike in time. A high exp signaler is quicker making calls, so sitrep spamming before later waves can be helpful.
If you pull off 25 of these you get a ribbon.