Website: https://forums.macrumors.com/threads/how-prerecorded-voicemail-message-to-iphone.2051253/
Alternatively, you could save yourself some time and hire an affordable professional. Smith.ai recommends Fiverr’s Voiceover talent. Starting at just $5, you can purchase a customized voicemail greeting, ready in just a few days. Many of our current small businesses and entrepreneurs have messages provided by Fiverr’s talent — we think they provide top-notch service.

Step 4. Begin recording your custom voicemail message by tapping “Record.” You can also listen to your recording by tapping “stop” and “play” before proceeding to the next step.
If your carrier supports the iPhone voicemail feature, you can retrieve and listen to your voicemail right from within the “Phone” app and call your contacts back by simply tapping their messages. That type of functionality is pretty neat and useful, because it saves you actual time. If not supported by your carrier, you will not be able to see the visual overview of your voicemail inbox and use any of the related features. Tapping the “Voicemail” tab in your “Phone” app will simply call the number associated with your voicemail box. So let’s get going with creating our custom voicemail greeting on iPhone! How to record your own Voicemail on iPhone Phone → Voicemail → Greeting → Custom → Record → Save