Depends on who responds. Some will just nod and inform the caller that they are indeed federal law enforcement, but others (who absolutely despise ICE as cosplaying wannabes) may decide that taking the actors into custody, even if only for their own protection if a crowd is forming, is the best course of action. If it happens enough, it should force DHS to direct agents to properly identify themselves, and even if not, some actual consequences for these thugs may discourage some from participating. And as others have pointed out, friction between law enforcement agencies is in the population’s best interest.
Except they aren’t targeting the one group they always said they would; masked thugs snatching people off the street.