Voicemail notification popups issue

Hi alpy21,

Thank you for contacting Apple Support Communities. Based on what you stated, it seems you are receiving notification for voicemails you do not have.

Since it changes with the cellular connection, you can try updating the carrier setting on the iPhone.

Update your carrier settings on your iPhone or iPad

Cheers.

Hey alpy21,

Thanks for the update. One thing would be to make sure you have the latest version of iOS installed. Even though there are some steps that you've already tried, we would recommend you follow every step and let us know if the issue persists. If it does, you may need to contact the carrier.

Have you recently changed carriers?

Are you using a VPN?

1. "Make sure that you have the latest version of iOS.
2. Check for a carrier-bundle update: Go to Settings > General > About. If an update is available, a message will ask you to update.
3. To check your cellular network connection, try to call someone. 
4. Make sure that you set up your voicemail. To check, dial your own phone number from your iPhone and leave yourself a message.
5. Call your iPhone from another phone and try to leave yourself a voicemail.
6. Go to the keypad and press and hold on the 1, or press the Call Voicemail button. This dials voicemail. Check if you have any missed messages.
7. On the Voicemail screen, tap Greeting. Then change your greeting to either custom or default. 
8. If you have more than one cellular plan on your iPhone, try switching to the other plan. Call the first number and try to leave a message.
9. Go to Settings > General > Reset and tap Reset Network Settings.
10. 
11. 
    
12. If you still need help, contact your carrier."

Set up Visual Voicemail on your iPhone

Have a good one.