I reckon the post of my saga is going to get buried, so I thought I'd post this up under a new heading so it might help someone else who finds themselves in a similar position. Here's the link to my original thread: http://community.giffgaff.com/t5/Help-Support/Mobile-network-not-available/m-p/16865408#M5571110
Very short version: If you are the legal owner of a phone, then it is a breach of contract for your operator to block it - regardless of whether or not your IMEI is on a blacklist. I sued giffgaff, won and got paid for a replacement handset, time and expenses.
On combing through the T&Cs for every major operator, there is no provision for blocking by IMEI. There are no statutory requirements either - it is done entirely voluntarily.
What seems to happen quite typically is that people buy a phone 2nd hand, it works for a bit, then winds up on the blacklist and stops working. Assuming that you bought it from the genuine owner then you become the new legal owner. Your network cannot block your handset, even if the previous owner reports it stolen for an insurance payout. That's fraud and is the concern of the insurance company - not the new phone owner.
[edit here to remove point contested further down the thread]
If you buy a stolen phone however, then yes - if it's blacklisted there's nothing you can do about it apart from return it to its rightful owner. The operator blocking the handset should
tell you who this is, or at least make provisions for its return.
The full version of my saga:
I was a customer of giffgaff for some years. I bought a phone in March 2014 from a genuine retail seller brand new in box and found it blocked in June 2015 (yes that's 15 months later). I discovered on ringing around that the phone was blocked by O2. They refused to tell me the reason and despite promising many times to unlock it, it remained blocked.
My contract for service was with giffgaff. On combing through the terms and conditions there is nothing there that allows giffgaff to refuse service based on IMEI number. I also went through the T&Cs for all the major operators - there is no provision to block based on IMEI number for any service provider.
I attempted first to contact giffgaff via the website, but I don’t think the agents I corresponded with had a good enough handle on the issue. I sent a snail mail letter to giffgaff HQ asking that either they satisfy me that their refusal of service was within the T&C's or pay for my new phone - some £250 ish. I threatened legal action if I did not receive a reply within 28 days. I eventually found a reply via the website message centre - after the 28 days and the response was not satisfactory, nor even appropriate to someone threatening legal action.
I used the money claim on-line service to start small claims court proceedings for breach of contract. https://www.moneyclaim.gov.uk/web/mcol/welcome
I asked for the cost of a replacement handset, and recompense for the time I'd spent trying to sort out the issue. Giffgaff did not respond to the court so I won by default. I requested a bailiff to collect the money which was paid in full.
So if you find yourself denied services by IMEI blacklisting despite being the owner of the phone - ask your operator to remove the block, pay for a new phone, or prove to you that the block is genuine. Funny note here - the letter I got from O2 cited "genuine reasons" for the block. This is ludicrous - you cannot accept genuine reasons that are concealed.
If they fail to comply - sue them. You don't need a lawyer to do so - follow the above link and the instructions. It's really very easy.