My iPhone 12 Pro shows voicemail only when I turn off cellular data on

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. Here's how you can check your voicemail on an iPhone and what steps you should take if you encounter any problems:

Checking Voicemail on iPhone:

1. Visual Voicemail: On most iPhones and carriers that support visual voicemail, you can access your voicemail messages directly from the Phone app. Open the Phone app, and you should see a "Voicemail" tab at the bottom. Tap on it to access your voicemail messages in a visual and user-friendly interface.
2. Traditional Voicemail: If your carrier doesn't offer visual voicemail or if you prefer the traditional voicemail service, you can access your voicemail by dialing your carrier's voicemail number from your iPhone.

Troubleshooting Voicemail Issues:

1. Voicemail Not Working: If you are experiencing issues with accessing or receiving voicemail messages, it's essential to contact your carrier's customer support. They can help you troubleshoot the problem and ensure that your voicemail service is set up correctly.
2. eSIM Users: If your iPhone uses an eSIM, ensure that the eSIM is activated and set up correctly with your carrier. Any issues with the eSIM activation can affect voicemail functionality.
3. Network and Coverage: Sometimes, voicemail issues may be related to network coverage or signal strength. Check that your device has a stable cellular connection.