Hi purahu,
Welcome to the forums :)
The only time I've ever seen the speaking clock as part of a billing dispute is shortly after a fault being resolved. I used to think that it was some sort of testing method to check a phone line, but the explanations in Jim's link make a lot more sense. I'll get the speaking clock fee refunded for you, whatever it may have been.
As for the 0870 number; I tried calling it and there was a pretty deafening tone on the other end, a bit like a fax machine or a dial up modem.
By any chance, do you have any alarm systems fitted in your property?
Matty
ex-Plusnet staffer. Any posts after 28/07/2017 aren't on behalf of Plusnet