Are these configs read-only on the phone or is it possible to manipulate them using the data here to de/activate any service?

You can trick iOS into running different bundles for your carrier up until the most recent beta (just fixed)

A very common thing to do in Russia for instance where WiFi calling isn’t available: https://github.com/ios-bundles/CarrierSIM

Depends on the OS. GrapheneOS can override a bunch of them: https://grapheneos.org/usage#carrier-settings-overrides