Any call that brings them in. You need to know what call not to use if you use mouth blown calls. That is generally the call they use when humans come into their field. Never use that one.
Why don't you get the "cawlin' crow" a decoy that takes batteries and does all the calling for you? they go about $40.00 and get the one that is flocked or buy one and carefully paint it flat black...do not get anything into the speaker and battery compartment idea and you will be good to go. You buy all the e-callers but if you live in a big crow area then all the other guys are using them too. This not bode good luck for you at all. I know someone who has competition and he sometime shears them out there....and no shooting...!
The cawlin crow can be found on ebay under sporting goods then type in crow decoys and look there. It will a lot easier using one when you got to walk out 400 yards to your blind.
Another item; if you have a roost in your area and it is really huge get a mile or so out from it-maybe even more as they will ignore any decoying efforts as the need to eat over rides attacking owls or joining very small groups here and there. And if you shoot up the roost they will go else where to roost and you could be hunting for a few hours looking for them and that is a huge pain in the bacon..follow me?
Good luck!