Setup voicemail on iPhone 8

Did your sister do the following: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Then see the following from Check your voicemail on iPhone - Apple Support

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.

the previous owner to my iPhone X did all the stuff to rid her data on this phone. When I go in to set-up voicemail there is NO option! When I call in to voicemail it asks for my password. ????! Frustrating!

Did you add a new SIM to the phone?

Did your sister do the following: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support ?

Did you setup the phone as described in: Set up your iPhone, iPad, or iPod touch - Apple Support ?

What happens when you open the Phone app and tap Voicemail at the bottom right of the screen?

eastnewyork wrote:

Do I have to add a new SIM to my iPhone 8? Thanks.

If you are not using the cellular plan that was on the phone (e.g., your sister's cellular plan) you will need to obtain a plan from a cellular carrier. They will give you a SIM if needed.