Like many of us, I regularly view this and other similar noticeboards and I'm surprised that I have never seen the following problem highlighted:
If you take out say a 12 month cashback contract with o2 and cancel it at the end of the contract, the bills will be as follows:
Beginning of month 1 - invoice for the coming month
Beginning of month 2 - invoice for the coming month including receipt for month 1
Beginning of month 3 - invoice for the coming month including receipt for month 2
Beginning of month 12 - invoice for the coming month including receipt for month 11
Then, here's the problem - If you have not exceeded your monthly allowance and have cancelled your contract, there will not be an invoice for month 13 and hence no receipt for month 12.
The retailers want to see that you have paid the 12 months of the contract before they give you your cashback, but it is not o2 policy to send out a final bill when the balance is zero(even though they email you to say they ARE sending a final bill).
If your retailer accepts downloaded statements from the service provider's website such as mobileshop.com, cpw, e2save etc, then you're ok. Others, such as OSPS only accept originals.
I've tried to get a final bill out of o2 for the last month of my contract and they've been absolutely hopeless.
My suggestion is to ensure that you use a chargeable item during the last month of your contract, hence ensuring there will be a small invoice for month 13.
As I mentioned before, I'm surprised others haven't fallen foul of this quirk.