iOS setup is personalized so your carrier shouldn’t be setting up anything for you. E.g when you startup you need to setup your faceid and other personal info, that a carrier can’t setup. The most a carrier can do is pre-configure the phone with a specific activation.
If they’re skipping that for users then they’re breaking the entire flow for transferring data , setting up payment and authentication etc…
I don't know how it is in other countries, but here in Japan, when you buy a phone in a mobile carrier shop, they set it up for you, which means any on-boarding that exists on the phone OS is skipped entirely.