The key piece of legislation that covers phone warranties, and those on many other devices, is called EU Directive 1999/44/CE. This states that any device sold to a consumer inside the European Union has to carry a warranty from the seller that the device will meet the quality that you would expect for such a device for a period of two years, and in that period any costs incurred in replacing the device are the responsibility of the manufacturer.
What about the nature of the failure? In the first six months, it is assumed that whatever went wrong existed at the point of sale, and therefore the customer isnt required to prove he or she was using it in any particular way. After that grace period, the seller can claim that the defect was caused by some action that was triggered by non-normal use of the device, though the burden of proof is for them to prove that this caused the unit to fail.