My new iPhone voicemail says voicemail not available

It is not an Apple issue. Call your carrier back and ask to speak to their next level support. Your Voicemail is on the carrier's voicemail server, and they have to provision your account for visual voicemail. They are the only one that can fix it.

Voicemail is managed by your carrier. You need to contact them if it isn’t working right.

I have contacted my carrier and they have no clue why it isn't working. They said it is an Apple issue. I had visual voicemail on my iphone 12 pro and once I switched to the 13, it disappeared.

You’re welcome; it should be easy to fix, if you can get through to them.