Agree with Scoutrider & others, that the best way to get them to come when called is to ensure they see you as a Good Thing in their life, by rewarding them well when they do come.
I don't think the cue's that important ~ my first horse responded to a whistle or "c'mon!", then I went through a clapping phase(
) ~ lost my voice, so would clap to 'call' them. I tend to just call my 2 current horses by name, although when they're in a certain paddock that I have to drive around to get to, I toot the horn & they're waiting at the gate by the time I get out. ...Sometimes I toot when driving along the road past them, just to confuse them...
. The primary 'cue' is them seeing me come to visit, so I've found whatever other 'cues' I use are more like attention grabbers.
The important thing is to associate whatever cue with the behaviour you want first. Eg. Start in a manner that you're highly likely to get the desired response regardless, such as Scout has described. I would just start out rewarding her for looking my way, attaching the cue as she does & as you reward her. Once you've done a fair few repetitions of that & she's eagerly looking towards you for a treat, then begin giving the cue before the behaviour, when she's looking away. If it instantly causes her to turn to you, great! Reinforce that & repeat a number of times before asking for a little more/further, etc. If she doesn't respond to the cue, go back to the primary lesson & reinforce the basic association with the cue to the behaviour some more. Do this any time during training; if she doesn't respond, retreat to a previous successful distance/difficulty level & repeat before carrying on more gradually.
You should also strive to reward her *at the time of* the behaviour(eg. As she turns to you, she finds the treat and hears the cue), as even 3 seconds later makes it more difficult for a horse to associate cause with effect. Obviously as you progress & increase the distance, it becomes impossible to reward her instantly or within a few seconds, but by that stage, after progressing gradually, the behaviour should be well enough established that it isn't an issue.
Once the behaviour is well established, I also teach them a 'hurry up about it' cue. Eg. If you want them to run & they're dawdling to you, or if they take their time to respond or such. Again, I start up close, set up in a manner that makes the Right thing the easiest most likely behaviour ~ on line. It is effectively refining the yielding to implied pressure of a stick or rope behind them, such as in lunging. When they understand your signals of 'speed up' on the lunge, start using that cue(vocal, if that's what you've associated with the behaviour, backing it up if/when need be by waving the whip out behind them) Start cuing this when asking them to come in to you faster(obviously this is a different position & if you're in front you'll be waving the whip beside them rather than behind. Then, again as previously described, gradually increase the level of difficulty as with repetition the horse becomes solid with easier degrees.
Originally Posted by Scoutrider
you may need someone to keep her out of your pockets between "calls".... If she starts just following you around, as before, you may need a helper, or perhaps move to a grassy enclosure so that she isn't quite as motivated to reward-grub between calls.
That is something that I teach first & foremost when training a horse ~ manners & basic 'rules of play', before I teach anything else. I will have already taught the horse that staying out of my personal space & being 'polite' is what works, and that 'mugging' me or otherwise being 'rude' *never* works to get a reward. Even when that's just after doing something Right.