Unlikely, I know, but worth checking the possibility of technology you might have around the place that could be making automatic calls. Normally, if this happens, the tech is set to act benignly and not trigger charges. But it’s nice to know what they’re doing all the same.
Sometimes TV systems (like Sky in the UK) make phone calls to authorise access to films and do other mysterious stuff.
Answer machines can ‘call back’ to check phone numbers etc.
Routers can also access your phone line.
Also (as you are no doubt aware) when you leave a call to go to answer machine the answer machine answers it on your behalf, so it’s logged as a ‘received call’ by the telecom provider whether or not you then pick it up yourself.
Good luck with chasing a more useful response from Orange.