Your business depends on customer engagement, and voicemail messages are a large part of that engagement. A voicemail is nothing more than a digital recording system, where messages are stored for you to retrieve later. Voicemail allows you to be able to connect with customers while you are away or during your off-hours, where they can drop off a message while you are gone. A voicemail greeting, on the other hand, refers to the message that your callers hear upon answering the phone.
Tap on Voicemail; From there, you can turn Voicemail on or off, set your Greeting as a default (or record a custom greeting), and turn Voicemail Transcription on or off. Record your own voicemail greeting. Note: Recording a custom voicemail greeting is only available through our iOS or Android app versions.

How do I change my voicemail greeting on Android? Open the Phone app . Press and hold “1” to call your voicemail. Enter your PIN and press “#”. Press “*” for the menu. Press “4” to change settings. Press “1” to change your greeting. Follow the recorded instructions. Access your main voicemail menu. Listen for instructions to enter your password. From the main menu press 3 for Personal Options. Press 2 for Greetings. Press 1 to Change Greeting. Press 1 to Record personal greeting or press 2 to select the standard greeting. Tap on Call settings and select Voicemail. Tap on Voicemail service and select My carrier or My operator. Tap on Setup, select Voicemail number and type in your voicemail number. Tap on OK in the Voicemail number changed popup.
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.