my voicemail is unavailable after update

Tap Voicemail, then tap Set Up Now. · Create a voicemail password. · Choose a greeting—Default or Custom; if you choose Custom, you can record a new greeting.

Set up your voicemail on your iPhone

Please contact your Carrier as VoiceMail is a carrier-specific feature --> Check your voicemail on iPhone - Apple Support

VoiceMail, whether it's visual voicemail or the traditional voicemail service, is a carrier-specific feature that depends on your cellular service provider. If you are experiencing any issues related to voicemail on your iPhone, it's essential to get in touch with your carrier for assistance.