How do I enable live voicemail on iPhone 12?

Short Answer: You will need to upgrade to a newer iPhone device or use an unsupported method to get iOS 17 onto your iPhone 12

Long Answer:The iPhone 12 is a slightly older handset and is not officially supported by iOS17. Third party methods may exist to get iOS17 working on that device however none are endorsed or supported by Apple and using any of them could void your entitlement to software and or hardware warranties applicable to your device.

Use Live Voicemail on your iPhone - Apple Support

https://gbtimes.com/can-i-get-ios-17-on-iphone-12/#google_vignette

Hello Marklouie1703,

Live Voicemail is not available in all countries. Here is the list: iOS and iPadOS - Feature Availability - Apple.

Alin S wrote:

Users device does not meet prerequisites for iOS17.

Recommend the user consider a device upgrade.

(updated grammatical clarity. I think this persons question is well answered.)

This is incorrect: the iPhone 12 is perfectly compatible with iOS 17. It is also compatible with iOS 18 (iPhone models compatible with iOS 18 - Apple Support.) which is the iOS version the OP stated in their question.

Users device does not meet prerequisites for iOS17.

Recommend the user consider a device upgrade.

(updated grammatical clarity. I think this persons question is well answered.)

An iPhone 12 supports the latest iOS 18.