I should heave prefaced my post with what I meant by bantam ducks, LOL.  I meant the Aussie Spots, the Mini Appleyards, etc...  I did not necessarily mean Calls, but it is interesting the lady told you that Annarie.  I am probably not the best person to talk to about Calls because I have not raised a lot of them in the past.  I do have a couple dozen Calls now though and I am really shocked at how quiet they are.  I have always heard that Call hens are so loud.  I never hear a peep out of any of mine (I have probably just been lucky so far though, LOL).
As far as the other bantam ducks previously mentioned that I have kept for a number of years, very few of my hens quack (certainly nothing like some of the larger breeds I have kept like Welsh Harlequins, Swedish, Campbells, etc...).  I certainly couldn't sex them as youngsters based solely on sound.  I still think that tameness has a big impact on how loud a domestic duck is.  I know I have mentioned that before and a couple people here told me I didn't know what I was talking about, LOL.  I have seen time and time again though that my loudest birds are the ones that were usually raised in smaller groups and are tamer individuals (hence someone that only keeps a small flock of pet ducks may notice a lot more loud quacking).  The reasons for this should be obvious to most people that have studied animal behavior.  Most of the birds that I have bought as adults (usually from larger farms) literally hardly make a sound.