I had this happen several years ago. It was a different plan, but it was AT&T. I had a plan that was 5 cents/minute with no minimum. Suddenly they started charging me $5/month. I called and raised hell, and cancelled my service and got the charge reversed.
A quick web search will turn up many alternative long distance services with low rates and no minimum. The one I currently use charges 3 cents/minute, with no minimum. Since I use little long distance, my bills are often a few cents or less. They let it go for months until it gets over $1.00, and then bill my credit card.
They also provide a toll-free number at no cost (unless somebody calls it .. then you pay the regular 3 cents/minute). I don't really need a toll-free number, but I got it because it was free. It's a "real" number too, not one of those where the caller has to enter another pin or extension number after calling the number.
Cancel your AT&T service.