Don't be fobbed off. Your statutory rights are that if you have an inherently faulty phone you are entitled to a replacement or refund.
If there is a complication because of a contract, then replacement is what you should have. It is more complicated under contract.
But sadly o2 seem to be treating everyone with the same contempt.
After 28 days it's repair only.
That is undermining your statutory rights.
Don't be fobbed off complaint to Matthew Key, the chief exec and more importantly trading standards who are looking into complaints about this 28 day rule.
The legislation to quote is the c. Supply of Goods and Services Act 1982. Sale and Supply of Goods Act 1994. The Sale and Supply of Goods to Consumers Regulations 2002.