T-Mobil Not Honoring Their Agreements

I have a similar story to tell.  I've been a customer for over 10 years. Last fall I was told if I switched plans and added a line that the phone for the additional line would be free via monthly credits.  I was told not to worry if I was charged for the phone on the first bill, the credits sometimes took a bill cycle to post.  Every month I have had to endure long phone calls in which they review the notes from prior calls and acknowledge that I am supposed to recieve a monthly credit to offset the finance charge.  Today was my 6th or 7th call.  Now they are saying that they can no longer give me complimentary credits.  I explained I am not looking for anything complimentary, I am just looking to have them honor the promise they made to me and stop stealing my time and money each month.  I am at a point where I want to send a bill to T-mobile for all the time I've spent on the phone with them.  I asked them to play back the recording of our last call where I was promised they would resolve their issues so I wouldn't have to call again.  Today they basically told me that they won't credit me anymore and there is really nothing I can do about it.  I told them that is called theft and they should review their call logs before committing to that course.  I'm supposed to get a call from their Executive Service team but I'm not holding my breath.  What, if any, good came from filing a BBB complaint?