I worked on a project for a major telco who regularly struggled with customer bill shock. Business travellers would go abroad, run up bills without knowing it (literally tens of thousands of pounds) and then complain that the provider never warned them or set a cap. Often the provider would have to eat the cost (it wasn’t massive markup, these were just the underlying trunk costs). We spent a long time building alerting and monitoring tools, which would even lock out your 3G dongle if you’d spent too much.
Unfortunately for us (but fortunately for customers) the EU introduced legislation to cap this spend and things have been pretty good since then in those parts of the world. I’m hoping UK customers don’t go back to the old days post Brexit. I do some business in Egypt and it was my first experience in a decade of actually having to worry about it and wasn’t prepared at all.