You only have untill the 29th of this month to get the upgrade for Free!

Good morning all... I checked my credit files this week, and was rather shocked to see a default registered by Orange, last year for £65. Background is this: I have been an Orange customer for over 15 years, and towards the end of 2014 my handset stopped working and I decided it was time to upgrade and get a new one. Simple, or so I thought. I called Orange from my handset, and was quickly upgraded to a new contract by the representative who told me that my new contract was with EE but that it was all the same company and nothing would change. My account was not in arrears. The EE account has been running since, no arrears, no notification of outstanding arrears, until last week when I discovered the default and outstanding balance. I called 150 from my handset, and was transferred to a call centre in India where I was told that I had not paid my final Orange bill, which was strange as I can't remember the phone representative mentioning anything about it when I upgraded and I haven't received a bill or reminder about it and of course I am still an EE customer, they know exactly where I live and I still have the same number. I asked to speak to a manager, was kept waiting for 40 minutes, then hung up on and had to repeat the same process again. I finally got through to someone who told me it was my fault, they had apparently sent me my final Orange bill with an amount for the first months EE bill and insurance. This is even stranger; surely I would've been cut off if I hadn't paid my EE bill, and this is beside the fact that I was told nothing would change when I upgraded my phone? Orange told me I need to pay the DCA, and there is nothing they can do. I have received no correspondence from any DCA. Reading around other threads it doesn't look good for having the default removed, and I am so frustrated, as my credit file would be clean at the end of the year and I was looking to get a mortgage. It doesn't seem right that Orange/EE should be able to do this, and I certainly would not have upgraded my contract with them had I known, but understanding that mobile agreements are not bounded by the CCA I'm not sure how to challenge it?