There have been people who got printed bills from their cell phone provider for every single kilobyte of data, each individually indexed and billed: https://en.wikipedia.org/wiki/300-page_iPhone_bill
The company had made a change of some sort where whatever EDI connection between the telco and the company stopped working. I learned this when and angry facilities guy came up looking for my boss’s boss to sign for a delivery. I was the only person there, so I did. 15 minutes later, two pallets of bankers boxes came up - thousands of single sided pages of itemized call details.
Really sucks when you carefully load 10 EUR of credit to buy a 10 EUR prepaid plan for the month and see 0,05 deducted despite being incredibly careful to not do anything that would incur a charge before buying the plan.
There is a pop up that says "Your carrier may charge for the messages used to activate iMessage and Facetime" You can choose to not activate and do it later.
There’s a field in there that configured whether that warning should be shown.
My memory, which may be wrong, is telling me that the first major version of iOS which included iMessage did not include the warning at all, and that it was added for non-whitelisted carriers (aka those which did not sell the iPhone) to prepare the user for the possibility that they will be billed, based on user feedback precisely like the comment to which I was replying.