Well I understand that some of the prezygotic barriers to reproduction are gone -- in the case of tincs and leucs, I understand that they occupy slightly different niches (tincs are much more terestrial), and that in captivity they are brought close together and that habitat occupation barrier is gone.
I also get that the mating behavior for these two species are very similar: the male calls to attract the female, the female appears and strokes his back, the male leads her to a egg deposition site, etc..
But the call thing still confuses me. From what I understand calling is a crucial part of frogs attracting a mate, and females preferably chose males with louder calls (calling is supposed to be a very energetically-demanding task, so there is a positive correlation between the loudness of the calls and the individual's fitness). But from what I understand female frogs are only supposed to respond to a certain type of call (differentiating it from all the other frog calls in the forest), and the calls of leucs and tincs just seem too different to me. Although I don't know how similar they sound to the frogs of those species...