The telephone systems have the feature named as Voicemail. VoIP is the best example of it. When a receiver is unable to pick up the phone, then the caller can leave the voice message on the receiver’s phone. In older telephonic machines, such messages were stored as a voice message directly, and the receiver can hear them later. In modern telephone systems, such info is being stored on the server of the service provider in the user’s mailbox. It is similar to email in which the messages are saved in the form of audio instead of text.
6. Contact Your Carrier For Support. If your voicemail settings still aren’t working, it could indicate an issue with the service provided by your network carrier.

Now, you will not receive any Voicemail on your Android. In case, if you still redirected to the Voicemail inbox, then follow the steps above in the app.
To record greeting, it is mandatory to have a strong internet connection. Alongside, even if the network connection is perfect, then switch to Wi-Fi to Cellular data or vice versa.