Keep trying. Sometimes it takes multiple calls or have other people call also. When you do call, get names, if the person you're talking to can't help you, ask to speak to their supervisor and just keep going up the chain until you get to someone who can. In CA I know the animal rescue people are very busy and sometimes it requires more effort on my part. If you are sure these horses are being neglected, can't get a response from your closest humane society/animal control people, call your local radio/TV station and see if you can get them involved. As Speed Racer said though, have all your facts straight and be calm but serious about the situation.