I was recently charged for a text to 6592324290, an international text, when I transferred my number to giffgaff. I didn't send the text and it was an unknown number.
Found the answer though, my McAfee Mobile Security sent the international text when I got my pin wrong when I changed my new giffgaff sim into my theft protected phone.
Hope that helps anyone else similarly confused.
A very useful tip, too often these unrequested charges are the result of either such legitimate software, malicious malware, or even just the basic function of things like Apple iOS which sends a hidden SMS to register Facetime/iMessage and eats 6p for people without a goodybag when a new sim is inserted into an iDevice