I do not want the voicemail feature in India. How to disable the voicemail in iOS 18?

you will notice that voicemail is a feature that works even if your phone is fully off or out of cellular service

so at the end of the day it's 100% up to your carrier if disabling it possible and not something your phone really control

In the settings, at the very bottom, there are Apps. From there, find "Phone” Make sure that the call forwarding feature and live voicemail feature are turned off.