No voicemail options when system language is English

I'm under IOS 18.4. My system language has always been English (United States). I was tring to reset my voice greetings but found the "Greeting" button on the top right in Phone-Voicemail disappeared (there was no button there at all). Then I checked Settings-Apps-Phone, and couldn't find any option for voicemail functions.

This was already weird to me. Then I was shocked when I switched the system language to Chinese Simplified, and found that all voicemail functions were back.

I find this not so reasonable, since the only change I made on the phone in between was switching its system language. Hope to find out why and how to actually fix this problem instead of keeping my phone in Chinese.