how to turn off voicemail on iphone completely

Think about what you want callers to get instead, though. With voicemail off, a call you don't answer just rings until they give up, or they hear a carrier message. Some people prefer that, but it can mean you never find out someone important tried to reach you.

There's no switch for it on the iPhone, because voicemail lives with your carrier, not the phone. Ask the carrier to turn it off on your line. Most will. On a lot of networks you can also dial ##004# and press call, which cancels the forwarding to voicemail when you're busy or don't answer. It doesn't work on every carrier, so if it errors out, just call them.