I think the best on the market is the Foxpro callers:
http://www.shootingbags.co.uk/products_callers.html
Though they are not cheap, £175 to £600 depending on model. That is a lot of money for fox shooting.
The U Caller also seem popular and starts at about £100 so much cheaper. You dont have the range of sounds of the foxpro and the sound level is not great
The call master is a little more but you can up the volume with an external speaker.
One final option is to use an MP3 player of some sort. I have downlaoded a selection of calls onto my phone, that is also an MP3 player, I then play these when out with the lamp and it does work, I have called foxes in closer or held foxes static with the calls.
But like all calls they dont work all the time.
At this time of year a vixen call can work well, if you follow the link below you will see a link to download a loop of calls and periods of silences, I have used this with some sucess. Place the call out in front of your shooting position, set it playing and leave it be, it is a 20 minute sequence.
http://shootingcommunity.co.uk/verminshooting.htm
There are also some othe sites with some good calls, search for Varmint Al for instance.
Jerry