Unexplained international text charges on iPhone with Tesco mobile

On my last mobile bill from Tesco mobile I was charged £2.20 for international texts I didn't send. There were 11 texts listed, all to different numbers in Peru, all sent at exactly the same time, but there is no record of them on my iphone. At that time I was doing nothing with my phone, although I had received an email from a known business shortly before and sent a text to a colleague in Newcastle shortly afterwards. Has anyone any idea what has happened.

[Re-Titled by Moderator]

Original Title: Mystery outgoing international texts