Interesting question thats for sure. There are some days when ducks dont respond well to calling and need more of a visual attractant, so that would definitely be a waste of time. However one could argue that if there is low light conditions you could bring some ducks in close to you overhead and get them to land, BUT you wont be able to get them to land infront of you in a good spot majority of the time unless you're hunting in a tight area.
If you're just starting out set a budget and make sure you get 1-2dz decoys (more or less depending on the size of water you're hunting) and get a baby mojo. You can be the worst caller in the world and even forego the call so long as the ducks can see your spread. But its always nice to blow on a call when you're bored.
With different calls, if you see a different species, either keep calling like a mallard, do a couple quacks, or just stop calling all together. Buy different calls when you start targeting species of ducks instead of any ducks and when you can ID them in the air.
To start off with buy yourself a $20 walmart call until you think you can effectively call them, not just make duck noises at them. Or if you want something better what I'll usually do is just look around until I find a sale on something then just google the name of the product with "reviews" at the end and you'll find a plethora of information.
For decoys stay away from the Canadian Tire Flambeau ones, they're junk. I use the final approach standard mallards which I payed $40/dz new plus the ACE 4oz decoy weights so it worked out to be about $70/dz plus tax. Since its pretty much end of season keep your eyes open and you'll probably find some really good deals on waterfowl gear.
Really if you're starting out, heres a quick list:
1dz decoys: $45
1dz weights(dont cheap out): $30
1 string of decoy rope: $5
1 decoy bag (flambeau canadian tire one works): $20
Baby mojo (find one on sale): $50-60
Call: $20
So about $200 for the perfect beginners kit.