Voicemail

i just got the iPhone 12 today how come my voicemail doesn’t come up in my phone app ?

Set up Visual Voicemail on your iPhone - Apple Support explains:

Set up Visual Voicemail

If your carrier supports Visual Voicemail, you can set it up with these steps:

1. Go to the Phone app, then tap the Voicemail tab. 
2. Tap Set Up Now.
3. Create a voicemail password, then tap Done.
4. Enter your password again to confirm it, then tap Done. 
5. Select Custom or Default. If you choose Custom, you can record a new greeting.
6. Tap Save.

Set up Visual Voicemail on your iPhone - Apple Support explains:

Set up Visual Voicemail

If your carrier supports Visual Voicemail, you can set it up with these steps:

1. Go to the Phone app, then tap the Voicemail tab. 
2. Tap Set Up Now.
3. Create a voicemail password, then tap Done.
4. Enter your password again to confirm it, then tap Done. 
5. Select Custom or Default. If you choose Custom, you can record a new greeting.
6. Tap Save.