Costco offer for BOGO not honored

I added 2 lines in first week if March as offer of BOGO from Costco Store Kiosk. One of the line was new and other was port in from another carrier.

Now after 2 billing cycle the T mobile customer care is testing my limits and says the BOGO is no longer valid. I had made a conference call between T Mobile sales person who sold the lines and customer care as well but the customer care lady was not at all cooperative and was unprofessional. Than the other guy from Customer care joined and said the issue will be fixed. 

Its almost 20 days now and when I call the customer care , I am back to square one again I have to explain the whole offer and situation. I am clueless how to deal with the situation. 

I ordered a new phone with new line , If I cancel the line will I be able to return the phone which was bought 2 months back ?

Can I escalate to Costco regarding T Mobile poor services ?