I believe coyotes have an outstanding sense of smell and hearing. These are animals that can hear a mouse moving under 2 feet of hard packed snow, or can hear our calls from many hundreds of yards away, and smell us from 500yds (I lasered one coming to my call, from downwind, and at 491yds, he turned and blew out of there).
Certainly coyotes can hear, and distinguish between, the unique sounds each caller makes, whether an e-caller or a hand caller. We can distinguish voices between people, why can't a coyote, which has even better hearing than us.
I use hand calls 98% of the time, and sometimes my old, wood barrel Weems call works the best, and sometimes my Sceery works the best. They are both Cottontail-in-Distress, and my calling sequence is the same, but one call can typically outperform the others on any given day. I just don't know what call on what day, until I try them.
By the way, have you ever seen a duck hunter with just one call on his lanyard?
As good a caller as the Foxpro is, when it's used by an inexperienced coyote hunter, one that doesn't know how, or where to set up properly, it can educate coyotes without a shot being fired, because the coyote smells the hunter.
Isn't it reasonable to think that the next time that coyote hears that sound from another Foxpro, that the coyote will be more cautious in the approach? And if that coyote has a 2nd experience of hearing the Foxpro and smelling a human, that coyote will be even more cautious, maybe to the point he won't come to that Fopxpro sound again?
Maybe 25 years ago, I bought the cheap Foxpro, brand new, with Turkey sounds loaded on it, for about half price. I bought the adapter to let me download other sounds onto the caller, and, at that time, Western Rivers offered free downloads of sounds.
For the next 2 seasons, I took that caller on every stand. I did not call a single coyote with it, but I did call in coyotes with my hand calls on the same stand. I suspect that some sound/noise got imbedded in the caller while downloading the sounds that coyotes could hear and I could not, even though they sounded great to me.