Your question needs an additional overview. If talking about setting up this function on the iPhone, I really doubt if it's possible. You need to go to the service center. I remember I faced the same problem on my Samsung S9. My friend told me that it's very easy to set up temporary voicemail messages on the Android system. But days passed, and I wasn't able to do it. Suddenly, I found
this default phone app for setting up voicemails while reading Twitter. One guy has been describing a similar problem. It's such easy to use the app, you can't even imagine it. Just a few steps and I was smiling and started to use this reliable program on my S9.