Custom voicemail message keeps reversing to default

I have an iPhone 10 and I've recorded a custom message for my voicemail, saved it, and replayed it, sot that seems to work fine. However, the phone keeps returning to the default setting, and that doesn't seem to work at all because when I called my phone to test it, I get no response at all. How can I keep my custom message from returning to the default setting? Also, does anyone know what the default setting actually says?