I've been quite successful with the Woods Wise Hyper hot Cow Call. Have called in both bulls, and stopped cows dead in their tracks. It comes with a good video as well as cassette tape. It's very easy to use.

I've also got a Hoochie Mama call or did have, left it hanging on a tree limb while relieving myself last year. Guys in our camp have had good luck with it. Just about all of them have the Woods Wise call noted above. Nearly everyone has enjoyed good success with it. You can do cow mews, hyper hot calls, distress calls and bull squeals.
Regards.