Okay, so it's probably been awhile since I've manually equipped/unequipped heralds and banners. Recently, I moved/removed/replaced stuff around, and somehow managed to kill my call-to-arms skill.
So here's what I did. I manually dragged my Swordswoman into the bow slot. I also made it a quickslot button next to my call-to-arms button. Also made sure my banners were clearly not equipped. Even left my character equipment window open just as a sanity check.
I then clicked on call-to-arms, and the following error message displayed:
"This skill is being barred from use"