There's not too much you can do to stop the cat from meowing when in heat but you can try some things that may or may not work depending on your cat.
First isolate the cat away from any other male cats and then keep it's litter box clean and also find something you can give your cat that is warm for it to sit on.
Also sometimes catnip will calm a cat in heat down and stop her from meowing.
Playing with your cat to distract her when in heat can also help as well as using Feliway.