If you use an app to manage your calls/voicemails - such as Robokiller - you may have granted permissions to have the app use its own voicemail services.
If you no longer use the app, you cannot simply delete the app and everything returns “back to normal”.
You will need to reinstall the app, launch the app, go to its “Settings” (for example, the Robokillers Voicemail Settings, NOT your main iPhone Settings app) to make sure you properly turn off “Robokillers Voicemail”.
I should also note that I had called Verizon to reset my voicemail (just in case). Verizon had reset my voicemail on their end, told me to restart my phone, and then call *86.
Please bear in mind, your mileage may vary as you might have used a different app than I did and/or have a different carrier; however, the overall process should be similar.
Hope this helps as I had just fixed my voicemail problem as I write this out on my phone.