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Well, Divya from the BT online chat service says that she has refunded my 42p.
We'll see when the next bill comes through.
When I asked why I had had this charge s/he said that I had an alarm service on my phone which was why this was charged. Needless to say I do not.
When I pressed further, denying that I ever had any such alarm service, it all got a bit confused and confusing. S/he agreed that 123 was the speaking clock and also agreed that phantom speaking clock calls were a known issue.
So I left it at that.
I do wish these help lines would not make stuff up. It just makes everything so much more annoying and confusing.