They should not be listening on a call to the USA in the first place without a warrant. The warrants are not so hard to get, and in fact they can be gotten retroactively. Ie. tap, then get permission from the court within 72 hours or be ordered to stop tapping and not use the evidence.

However, you are not allowed to use evidence from an illegal tap to get a legal tap, of course.

We could catch more terrorists if we disregard all those annoying civil rights and that's worth it, is that what you're saying?