The reason why you were not called in the first place is the one you mention: your war with Ferrara (Even if Ferrara were on the Protestant side, one of you would not have been called, because you cannot be in two wars on the opposite side at the same time. What remains unclear (at least to me) is how the one who is called is chosen).
Second, afaik, you can be called only when the war breaks out.
If you want to joint, you need to ally Austria (probably impossible if they hate you), enforce peace (not sure that it works with the league war, never tried it - but in your case also probably impossible as they hate you), or intervene as a GP.
Thanks to all who replied.
@ Hermithill, I think you are right on the reason for the lack of initial CTA. It is just so very puzzling that no additional CTA takes place once I am at peace! Quite different from normal wars.
FYI, I actually increased my relationship values with Austria in the savegame out of curiosity - and you CAN use enforce peace on a League war leader. Pomeria refused and I joined on the Catholic side.
Also weird: there were two GP's on the protestant side and none on the Catholic side - but I could not join as GP intervention since at least 1 GP apparently needs to be involved at each side.
@ True Heir Of Rome, I was the ruler of the Kingdom of God on Earth, the armoured fist of the Mother Church - of course I had to join at the Catholic side to help my brethren!
And apart from the role playing, I just enjoy league wars since they are so very chaotic compared to the clinical wars of choice. France can be taken apart at some point in the not too distant future anyway.
@Bibor,
I waited a few years on my original game and was not called in. So I went back a few years (no Ironman for me - I enjoy experimenting too much, and especially switching to a new country once I have "won" with my original tag.). Finished my Ferrara war halfway through and waited. Within the year, the League broke out and I was CTA-ed.
Success!
(and then we were smashed to bits since protestants with France and Poland outnumbered us good guys 2.5:1. But we don't mention that detail in the Vatican history books).
So many hours playing EU4 and I still learn new things. Such a great, great game.
-von Schmidt