I can’t set up greeting for voicemail on my iPhone 15 Pro

Voicemail is a carrier feature; contact your carrier for help with it.

But a couple of things to check:

• You need to be connected to your carrier’s network
• You need to have Cellular or Mobile Data turned on

Try turning off Live Voicemail in Settings>Phone>Live Voicemail

Hope you can find a solution on this thread

Error 1034 when trying to set up voicemai… - Apple Community