I may be wrong, but as I understand it they have to replace the phone if faulty when you 'first' take it back (believe this applies to any purchase, subject to confirmation that the goods are faulty). However once you accept a repair they are not legally obliged to replace it. Morally perhaps but not legally. If I purchase something that is faulty I always request my money back and then (if I want to) repurchase the item. That way I am fully covered for the new purchase. I am sure if this information is wrong someone will point it out.