You could get billed for calls you never made every month like Bt use to do to me and say I must have dialled them, until I seen an article about the same thing on watchdog and pointed that out to them, then they decided it was a "hitch" in the system.
I had a lovely letter of apology and refund, so I replied with my own letter that said "I'm glad you finally believe me after you have called me a liar for 8 months, and yes the problem is now resolved becuase I am switching provider"