Just found this thread. I've had a small number of calls on my bill this month that I 100% have not made. Too early in the morning for one thing, but more importantly, we simply don't use the landline. As every family member has a mobile with unlimited minutes, there's no reason to.
Had a bizarre conversation with customer services though. They seemed more interested in offering me a new package. Still not resolved the issue.
Apparently the calls must have been made from our landline, because the computer says so. My handset says otherwise and as the Post Office have recently had to own up to, computers aren't always right!