AT&T
Just to call them out, since they deserve it, AT&T was sued 2 years ago for poor security allowing a SIM swap to occur, AFTER the customer TOLD AT&T someone was trying to take over their account and AT&T claimed they increased security on his account. SIM swap occured anyway, and whoever stole $651,000 of his cryptocurrency. It's awful to WARN your phone company and they STILL can't put your account on some kind of proper lockdown. T-Mo was also sued, 2 people lost $650,000 in currency total, although no mention of those people warning T-Mo ahead of time. These were JUST in 2018.
In the US, Sprint still won't let one just freely swap SIMs between devices (T-Mobile just bought them, but up to that point they were still not fully upgraded to 4G, so it was a weird mix of 4G LTE SIM-based authentication and IMEI from the still-in-use CDMA network.) Verizon Wireless went from CDMA to 4G LTE, so they now fully use SIMs, but they are more or less competent compared to AT&T so there have not been these kinds of complaints with them.