Navigate to your call list and add the number you want to avoid as a contact.Navigate to Contacts and select the contact you just added.Inside the individual contact, tap the three-dot menu icon.Select Route to voicemail.
Voicemail access and the initial password should have been set by your carrier or service provider when your device was first activated. But keep in mind that some carriers don’t use passwords when it comes to voicemail. If you don’t know if this service is enabled or you forgot the password, you should check in with them first. If you see the contact list opening, tap on the phone dialer icon located in the bottom-right corner.Once you’re in keypad dialer, touch and hold on number 1 key. Now one of two things can happen. One, if you the service is enabled your phone will dial the voicemail number and prompt you to insert or create a new password. You might also be asked to record the greeting message at this stage. And two, in case your carrier didn’t configure this service when your device was first activated, you’ll be prompted with a message to insert your voicemail number. If the first scenario is the case, follow the instructions provided by your carrier to finish setting up your voicemail. In the event that you’re prompted to insert your voicemail number, tap on Add Number. If you don’t know your voicemail number, call your carrier and ask for it or search online for something like “voicemail number + *YourCarrierName* + *YourCountry*”. Now follow the instructions provided by your service provider to finish setting up your voicemail. Note: If you bought a used phone or you recently changed the service provider, chances are you have the wrong voicemail number set up. In cases like this, open up the App drawer and tap on Phone. From there, tap on More and select Settings. Once you’re in Settings, scroll all the way down to Voicemail settings and tap on the entry. This will open a pop-up in which you can change the voicemail number.

12. "Hi, you've reached [company]. We're available by phone from [hour] to [hour] [time zone] Monday through Friday [optional: and from hour to hour on the weekends]. You can also contact us by going to our website, [URL], and live-chatting or emailing us. If you'd like us to call you back, please leave your name and number after the tone."
If that doesn’t work, you may need to either live with the voicemail notification just sitting there, or you may need to try a full reset of your iPhone. It is a step of last resort but can fix most problems with iOS so maybe worth a try. Just remember to back up your phone if you decide to give it a try.