Which calls are working for you please? So far I have had no joy with mine.
Sat at work I can't think of what they're called, but the most effective so far have been ..
1) Side button back and bottom right call button .. sounds like a puppy with it's nuts shut in a door.
2) Side button forward, call button top left. Mousey sort of squeaking like someone silently running with a typical builders wheelbarrow.
3) Side button forward, call button second down on the right. Sounds like the biggest bunny in the world being killed slowly by a stoat, again, and again, and again.
#2 was the one that made a fox try to eat the call from the top of a fence post after I changed to that once I saw the fox at about 400yds having been using #3 previously.
I think the key is not to just switch the thing on, turn up the volume and sit back in expectation. Volume needs to drop as Charlie approaches, and changing calls will often tempt in those that may have heard, but didn't like, the previous call.
Put simply 'Keep playing around with the thing. Foxes are nosey as well as hungry so you'll tempt them in sooner or later.'