Calls are learned, from the parents, but they are essentially just markers that allow birds with similar bill structure to find each other. Since bill size and the structure of the palate allow a crossbill to forage very efficiently on one narrow size range of conifer seeds, and since the resource is patchy, it is advantageous for the birds to travel in a group with other birds that all have similar preferences. That way the flock can quickly assess a tree, and an area, and move on if their preferred food is scarce. A crossbill that switched call type would be at a serious disadvantage, since it would end up with the wrong group, foraging in trees where it is less efficient. Detailed studies that have looked at this question have found call-switching to be extremely rare, as one would expect logically.