I can not erase my voicemail or retrieve them

You need turn On your voicemail box.

Then call your voicemail box number -> xxxx to listen to the main menu.

Every Carrier has their own main menu.

An example here:

1.

Choose 2 for the personal settings of your voicemail. 

Choose 3 for PIN code and choose a 4 digit code. You can also change your pin code here and indicate when your voicemail should ask for the pin code.

FYI: if you enter an incorrect code 3 times and you will receive a temporary code via SMS. You can change that later.

2.

Unheard messages are stored for 8 days. Messages you've listened to will be gone after 24 hours. Longer storage is also possible. Press 5 while listening to your message and it will be saved for 7 days.

Which voicemail box number you should call depends on your carrier, the voicemail settings are controlled by your carrier, so contact your carrier  .

When you get a voicemail you will get a sms message from your Carrier, so you can listen to your voicemail messages.