I tend to check my phone bill (Talktalk) and have found odd charges occasionally but nothing worth pursuing, probably someone else in the family anyway. I used to get quite a few to something listed as "services" not sure what that means. That is meant to be 24/7 free calls excluding mobiles and certain other numbers. Are you with Talktalk? I think you mentioned it in past but possibly wrong.
HTC U12 Plus review: Hands-on