Sometimes filters get corrupted and everything starts acting weird - I suspect that’s what you’re seeing.
There is a workaround for clearing broken filters:
From the Calls list view, go into that view and change the URL to this:
http://yoursite/index.php?action=index&module=Calls&searchFormTab=advanced_search&query=true&clear_query=true
you have to do this for each user.
Tell me if that changes anything. You should be able to add more than one filter, giving them different names, and also to edit existing filters easily.