Getting over charged by T-Mobile... and not getting any help!

I cancelled one of my lines 7 months ago, been getting charged for both lines all this time. I had to waste several hours on the phone with T-Mobile to get a refund. Meanwhile, they charged me again for both lines today. Hundreds of dollars charged and hours wasted. Totally unethical, they admitted it was their fault and gave me excuses for not being able to refund the money. After several on the phone I got them to refund the difference to me. On the phone right now trying to get the refund for the latest payment which was charged a few days early. Escalating to a manager now, they are wasting more of my time. Never do business with T-Mobile. Can't wait to port my number and never deal with this again.