Dear Sir/ Madam,
Ref: [contract number]
On [date] I bought a [make and model number] mobile phone from you. I have discovered that the phone has the following problem [add details].
The [Consumer Rights Act / Sale of Goods Act 1979] makes it an implied term of the contract that goods be as described, of satisfactory quality and fit for purpose.
As you are in breach of contract I am entitled to have the phone [repaired/replaced or refunded] and I would request that you confirm that you will do this within the next 14 days.
I also require you to confirm whether you will arrange for my phone to be collected or will reimburse me for the cost of returning it.
I look forward to hearing from you