If the females do not respond to some but respond to others and the two group's calls are different, then logic dictates that the females currently respond to only one call. If the communities were at one time combined then the calls must have been the same. It is easy to determine if the two groups have shared genes in the past.
You can speculate that the females responded to two different calls if you want but that would not change the speciation event.
"Speculation. Why is this kind of stuff given such high credibility by evolutionists? Can it be because it aligns with what they already believe?"
You speculate very well, however you did not ask why evolutionists believe what they believe. Is it possible your speculation is backwards and we evos believe in evolution because the evidence has such high credibility?
Just because they were living together in the past doesn't mean the calls were the same. It's speculation. The business about genes is a red herring. Are you suggesting you can tell what kind of mating call they had in the past, based on DNA?