Hello Mariekats,
First, please be sure that the process outlined in the "Set up voicemail" part of this link is being used to set up voicemail on the device: Check your voicemail on iPhone
Set up voicemail
The first time you tap Voicemail, you’re asked to create a voicemail password and record your voicemail greeting.
1. Tap Voicemail, then tap Set Up Now.
2. Create a voicemail password.
3. Choose a greeting—Default or Custom; if you choose Custom, you can record a new greeting.
If you continue to experience an issue, please check to ensure your carrier supports Visual Voicemail.
You can select your country or region on the following link to learn more about which wireless carriers support which features: Wireless carrier support and features for iPhone in the United States and Canada
Thanks!