Calling in a bull is not that hard if you practice your technique, and you are in the right place at the right time.
Calling in a legal 6 point "herd" bull is not so easy, as a herd bull may answer your call, but if he thinks you're far enough away that you're not a threat,
he might answer your call, then round up his cows and move away.
If you get an answer from what sounds like a herd bull, you should move in quickly and try and get close, (so that he considers you a threat)
then he might leave his cows and challenge you.
Try to be out there just before and just after a full moon,
and try to be out there just before and just after bad weather,
you will increase your chances if you do.
... and keep in mind that animals are never far from water, especially after a hot dry summer like this past one,
they will start to head towards water at dusk, and start to move away from water before dawn.
Good luck!