T-Mobile Loyal 6 year customer double billed and treated bad Internet

I have been a customer of T-Mobile's for 6 years. Paying on time every month. The past few month my service on the phone was very bad. I had no signal at my house. I made 4 attempts to get the problem fixed. It was never fixed. I decided it was time to change providers. I was not under a contract due to long standing service. My last bill was due to be paid,I shut my phone off a couple days before the due date. I was sent another bill a week later for 2 months of service totaling $122. I called and asked why do I owe 2 months now? When I have paid up to date every month. The reply I got was "you can pay it or be sent to collections." Very bad attitude on top of that. I said "I will pay what I owe and nothing more." They are trying to double bill me for my last month. At this point, I have decided not to give them a penny. My son,father,step mother,daughter,and friends have T-Mobile. We have all decided to drop them. Any class action suits I would be happy to join. Please never consider T-Mobile
My new service provider is Straight Talk. I am very happy with them.Phones are available at Walmart.