However, some cellular service providers allow users to access and update their voicemail by calling their own numbers from working landlines or phones on other wireless networks, so users whose accounts include such an option and who have access to alternative service could keep the voicemail greetings updated even when their own mobile phones cannot connect to cellular or internet service.
Tap "Call Settings," and then tap "VoicemailSettings." Tap "Voicemail Number." Type the voicemailnumber into the box or edit the existing voicemailnumber. Tap "OK" to save your changes. How do you check your voicemail from a different phone? Call your 10-digit wireless number. When you hear your voicemail greeting, press the * key tointerrupt it. If you reach the main voicemail system greeting, enter your10-digit wireless phone number, then interrupt your greeting bypressing the * key. Why is my voicemail unavailable on my iPhone?

In many cases, an update to your carrier’s voicemail app or settings can resolve the issue, but don’t forget to call your voicemail number to check if it is set up correctly. Once you’ve set up your voicemail, you’re free to switch off when you need to. There are other ways you can stay in contact, however.
If you have a visual voicemail applications installed on your Galaxy S21, you can alternately use that app to record and set a greeting instead.