A survey of second-hand phones by mobile phone forensics company Disklabs (reported in The Guardian) found over half of them contained personal information that could be retrieved.
The survey, which involved the analysis of 50 handsets bought second-hand via eBay, found security information - including the previous user's home address, credit card numbers and PIN codes – on 26 of the handsets. 60% contained telephone numbers on a call log, while porn was found on nine of the devices. The International Mobile Equipment Identity (IMEI) number had been changed on nine of the handsets; a process that's illegal in the UK but suggests the handsets may have been been lost or stolen.
Simon Steggles, the owner of of Disklabs, recommends that consumers carry out a 'factory reset' before selling a mobile phone.