How do I make my custom checkbox accessible?

I'm trying to implement more accessibility to the website I'm working on. I have a custom checkbox inside a button, followed by a list of checkboxes. When I'm using a screenreader, I expect the screen reader to tell me whenever a checkbox is unmarked/marked. This works for all the checkboxes, except the one inside my button. The purpose of the button is to mark/unmark all other checkboxes in the list (this is done by JavaScript). Everything works, except the screenreader is not saying anything when I unmark/mark it.

Here the code:

<button id="checkAll" class="btn btn-default checkAll checkAllInit" 
data-target="#allSenders">
<span class="icon icm icm-checkbox_select" role="checkbox"></span>
<span class="text">Avmarkera alla</span></button>

What follows is basically a list of checkboxes, which all can be reached and understood by keyboard navigation and the screen reader. It's just the button that won't give the correct feedback.

Any suggestions appreciated.