I have noticed that large companies in general are pretty rough with their customers these days. I had a new, single business line (land, not wireless) installed by Verizon and have had nothing but issues with their installation, setup, service and billing. I have had to waste hours of my personal time to get it all straightened out only to find that they slammed me for long distance. Even getting that worked out was a really big hassle.
As far as the wireless problem you described. Even if you were making payments, if you officially cancelled the lines under contract, they probably have the “legal” right in their contract to charge you an early termination fee. Sometimes it’s cheaper to reduce your plan to the minimum monthly plan and pay it out than to terminate and pay the fee. Go prepaid next time.
I hope it works out for you. They have an army of attorneys, so if you fight them, go to small claims court where you can defend yourself.