As has been said your basic rights are contained in the 1979 sale of Goods Act, as amended. In 1994 section 14 was amended, up to that time goods had to be of merchantable quality a term with uncertain legal meaning, in the 1994 act this is changed to satisfactory quality which is far more straightforward. Section 14 also requires goods to be fit for the purpose for which they were intended (a mobile phone is to make telephone calls to others). If goods fail in the first 6 months you may presume they were faulty at purchase and it is for the retailer to prove otherwise, not you. They will probably claim that as you used it for 4 months this shows it was not supplied in a faulty condition, do not accept this argument (it was supplied in a condition to fail in 4 months)
But you must act promptly, I suggest you write to the retailer, head your letter “letter before action” refer to your earlier contacts and remind them that under the 1979 act, as amended, goods must be of a satisfactory quality, suitable for the purpose and as thegoods have failed after 4 months it is for you (the retailer) to show they were not supplied in a faulty condition, send by recorded delivery and keep a copy, the courts do not use recorded delivery they use normal post and service is considered made by the second day, but I would still use recorded delivery. Give them 30 days to replace or refund the cost of your telephone (your choice), 30 days is a reasonable period and English courts like reasonable behaviour, and if they do not agree you will commence action in the county court. Your letter should make them sit up and take notice and will get you talking to the right people but if they will not agree start your action. Good luck.