Practice is king, but biggest issue some have with calling is, calling when you don't need to. If the birds are coming to your set up, DONT CALL!! all you can do is mess it up. Once they start not looking interested, then call. Read how the birds react, if they like the calling, call more, if they don't, stop calling. Exact same thing goes with flagging. Some days birds love the calling, just a couple clucks the whole way down, sometimes they don't like.
This is all just my experiences, I in no way have geese "figured out". HAhhahah.