New charges, no notification

We've had the exact same business service for 10 years. 2 business phone lines and starter business internet. I even have the original paperwork from when we signed up. So imagine my surprise when I saw we have a new charge of $5 for voicemail, that has never been on a bill ever in the past 10 years. I thought I missed a notice explaining this new charge. I didn't. No notices on any past bills. I contact Comcast through the Chat feature and am told that it was from an "internal audit" and we should have always paid it, but were not charged. So I pull out my original paperwork and see Voicemail listed and a cost of $0.00 next to it. And see old bills where under Business Voice is clearly says that enhanced voicemail is included. So no, we never paid for it, and never should have paid for it either. But now they're charging for it AND didn't issue any notice about the charge? I mention to LaQuoia in our chat that this seems a bit underhanded to me and all of a sudden my chat box disappears and I can't get back to it. Hmm.