AT&T does not honor its promotional BOGO offer.

Mid december 2015 i was given a promotional BOGO offer by customer retention team.
I was told that they will be charging for both the lines for 3 months and on 4th month i'll get the credit and later on it will be just one phone charge.
It NEVER happened and as of now i have called them 14 times since March 2016 but no resolution....
Their argument is that this offer was only for the new customer or if you upgrade your line and i did neither.
My question is that if i was not eligible how come they offered me this promotion at first place???i originally had no idea about this promotion and never asked for any promotions but rather ATT rep. gave me this offer upfront when i called them in dec 2015.

If this was a mistake from their side why should i pay for their mistakes...ATT makes the terms/conditions ....ATT gives the offer and later ATT denies the same offer...how silly is that.....

I had better offer from T-Mobile but since i was given this offer i decided to stick with ATT and this was biggest mistake of my life...they are the liars and cheaters...i dont know where to escalate this issue.......