Since I could not afford a Foxpro caller and just got a new Android / smart type phone I would utilize it as a fox caller . I down loaded various sounds off "www.varmintal.com/ " got myself a little 2 x 2 watt speaker off the bay ( item No 8808993616435) the speaker is small and surprisingly loud for its size and good value at £13.
The calls I have found good at are a mix of cotton tail (rabbit) and jack rabbit (hare) with 1 or 2 minute silences between the calls , mouse squeaks are also good on low to really draw in shy foxes I use this in conjunction with hand calls and have had good success. I also found setting the caller up away from where I am positioned and let it run is very productive as well .
i have learnt to put your phone on silent or in the middle of bringing in a fox you will get either your ring tone or text message sound echoing across the fields , this can do two things make you look a fool and frighten off the fox.