One of the best and quick fix to this issue is to turn off Wi-Fi for once and switch to Cellular data connection and then try to change the default greeting to custom on iPhone.
The automated personalized greetings allow you to quickly create a voicemail greeting of your choosing that gets personalized to each of your callers. You no longer have to take the time to record individualized greetings, and, you no longer have to use your own voice in the greeting (it’s true, some users simply don’t like to use their own voice!). Never end up like Jason in this vidéo :

While email and text support saw an increase in recent years, many customers, partners, and potential hires still prefer to call your company directly. When writing your voicemail script, include basic information such as a short greeting, your company name, an invitation to leave a short message, and the time frame in which the caller can expect a return call. If relevant, you may want to include your office hours, extensions for company departments, and the contact information for your office manager or HR department.
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