Short story: for all of the trouble, a basic foxpro for $2-300 is the way to go.
Like many people thinking about getting into predator hunting, I figured I already have a waterproof bluetooth speaker and there are lots of free sound files and apps out there, how hard could this be? My first diy caller went something like this: arrive 30 minutes before legal shooting and set out Bluetooth speaker. Press the ON button....BEEP! Oh sh!t, that sure was loud, I bet every coyote within a square mile just heard that!. Get settled in the fence row. Pull out my phone and link my phone with the speaker. BEEP! Sh!t, again!. Ok, will wait quietly for legal shooting time. 30 minutes later I am good to go. Load my rifle quietly, remove scope covers, time to play the dying rabbit audio file...PLAY....phone produces crappy squealing noise...Sh!t, why is the sound coming from the phone in my hand? Oops, Bluetooth speaker shuts off automatically after 30 minutes.. Run out into middle of field to turn the speaker on right at prime time. Speaker BEEPS again! Ffs. Guess how many coyotes I saw that morning?
Turns out there is a reason why people spend $2-300 on a fox pro.