Changing Voicemail greeting

Hi!

Suddenly, Siri's voice has started answering my voicemail. I've checked the instructions on changing voicemail greetings on the Apple Support Website (Change your voicemail greeting and settings on iPhone - Apple Support (CA)), and it mentions a "Greeting" button in the Phone app. However, as you can see in my screenshot, it's not present.

I appreciate your help!

Chris.