The topic of unexpected call charges gets regular mentions on the forum.
Do your unexpected charges involve 09 premium rate numbers? There have been mentions of those in the past on the forum causing large unexpected bills where the customer reports no knowledge of the calls.
Also, breaking down the phone numbers on the bills can show that the numbers being billed, or the call durations or times of the calls or the rates being charged for a particular number are inconsistent with each other or the published rates which should be charged.
Unfortunately, these topics usually swiftly disappear off to private message and an explanation or outcome is rarely published on the forum.