Folder \SquareEnix\FINAL FANTASY XI\sound\win\se\se000 contains the call files. Calls start from se000017.spw, which is the <call>, se000018.spw is <call1> etc. Last call file is se000038.spw, <call21>.
First move the files of the annoying calls you are going to replace to some backup folder. This way you have the old calls safe in case something goes wrong or you want them back for some reason. Personally I couldn't stand those calls 0-5, so that would be files se000017-22.spw. Call21 is more pleasing to the ear, so take as many copies of file se000038.spw as you needed to replace. Then just rename the copies to match the ones you moved away.
Test calls in game and you should be able to keep your speakers on in <call> happy parties from now on.