I feel sorry for the victim described in this report on Moneywise.

The resident of Florida went on a 3 week vacation to Switzerland, and came home to a mobile phone bill from T-Mobile for a staggering $143,000! This could not happen in Europe.

In the past, before the law changed, I was caught out by an €1,800 roaming charge while on a ski trip to Austria, but then my mobile operator cut off my roaming service until I paid the bill. Since then, EU law has been changed so that, for EU customers, roaming mobile phone usage within the EU costs the same as in your home country. In addition, if I roam outside of the EU (e.g. when I visit family in New Zealand or the USA), my operator is legally is obliged ("obligated", to US readers) to cap charges at reasonable levels, and then cut off service, and to send warnings when charges come close to the cutoff limit. One of the major impacts of Brexit was that UK residents no longer get EU roaming at the same rate as when at home (but you did it to yourselves, so you get no sympathy from me).

I don't understand why the USA doesn't have similar legal protections against roaming charge shock. Most roaming charges are anyway a scam; telephone companies pay for traffic at wholesale rates (if you want to get an idea of how low these are, compare the cost of a call over Skype with the same call on your mobile phone), so most of what you pay when roaming is pure profit. When EU roaming was introduced, all the mobile operators complained bitterly, but none were driven to bankruptcy, and all continued to make profits.

Admittedly, the Florida man with the enormous bill could have been better prepared for his visit to Switzerland. When I visit New Zealand, I always get a local prepaid SIM card, so that I pay minimal roaming charges (just for incoming calls on my German number). Most recent mobile phones can hold two SIM cards (plus the option of eSIMs - no physical card involved), so there are no technical obstacles.