T-Mobile has been overcharging me for months

We have been with T-Mobile for 19 years, but since the Sprint merger, everything has gone way downhill, causing us to look elsewhere. The last straws are... 2 "free" upgrades where nobody will credit the payments back to us, AND, bait and switch with internet, charging $150/month for non-unlimited instead of $50 for unlimited, THEN, sending us a second modem, so they can charge us for equipment and service we can't even use, since they don't allow 2 modem lines at one address. We are trying to track down ANYBODY who can help us at T-Mobile. Somebody said call 611. We did, they promised refunds, then gave us a measly $20 credit (instead of the $300 or so they overcharged)... and the new bill is still trying to charge for a non-existent line and unused service.