Can't set up voicemail on my iPhone

Had this same issue with new iPhone 15. Solved it by running Phone app, going to the keypad screen (for manually dialing a phone number), pressing and holding down the "1" key and that takes you to the voicemail set up from AT&T. It pulled in the greeting off the old iPhone.

Error 1034 cant ser up greetings in my voicemail

Thank you. This resolved the issue. Hold 1 on keypad. 👏🏻