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I've just signed up to say i've experienced the same issue.
I received the first text from 64404 on 6th December, aruond the time O2 network went down. i received the message again on 5th Jan which made me concerned and i looked online and lo and behold, there is a charge of £2.50 for both text messages.
The online chat is no help, you're best off talking to the O2 customer service and they can put a block on the number. I will update next month if the charge has stopped.
The fact that this has happened to various people at the same time when the network went down is concerning. This needs to be rasied with O2 and they need to investigate how this has happened.