Free voice line

Adding a line in October at Tmobile authorized retailer and was told that I can add an additional line free of monthly charge as a promotion. I added the line and come to find out from Tmobile that my plan doesn't qualify for the promotion. Tmobile retailer now says they are there only to advise and doesn't have the final say. This is confusing as a customer. Im a light user of mobile phone and switch only because it is free.