There is absolutely no reason for carriers to need to do anything at all, else it would not make sense that I can change the SIM card out and voila I’m on a new carrier. The manufacturer is the one doing the testing, and the carrier simply provides the infrastructure.
I feel US perhaps has a different experience, because of their ridiculous CDMA infrastructure. Is it Verizon or what, where it’s not a SIM card but the phone itself that’s setup to a specific carrier?