Cancel 1 Line

On two separate occasions I spoke to a customer service employee to cancel one of two lines I have.  The first one said, "It makes me very sad to know you want to cancel a line."  That was sweet.  It made me sadder later on to find out that despite this man assuring me that the line was canceled, it was not.  A few weeks later I tried again.  This time the employee got right to the point, expressed no emotion, and said, finally, "Done."  That made me feel better, until the following billing cycle, and I was charged again for the line that was supposedly canceled!  I am now out $70 dollars for a line I do not use.  I want to use chat or email to verify my experience, but T-Mobile only allows phone calls.  This smells like fish to me, rotten fish.  What is it going to take, a class-action lawsuit?  If I were living in the U.S. right now I would go to a local T-Mobile office and raise hell, but I am living in Mexico.  I want to keep their excellent international plan, but only on one line.  I am at wit's end with these people.  Just filled out one of their generic requests for my opinion of their services, but I doubt anyone will read it.  I have a lawyer friend.  Maybe that is the only route … a nasty certified letter from a lawyer.