change the sound of the greeting

On the iPhone, go to the Phone app, tap on it, then go to Voicemail. If your carrier provides the ability for a personal greeting, you should see Greeting in the upper left corner. Tap on that, and you should see the ability to select Default or Custom. Then under custom a slider bar should appear and you can Play the current greeting, or Record a new one. If you do not see Greeting or the ability to record a Custom greeting, then contact your carrier.

I am not aware of a way to do that, unless your carrier has something. The default greeting is a computer generated voice that just says your phone number and that number is not available.

Contact your cellular carrier.

Voicemail is a carrier feature.

They should be able to walk you through changing your voicemail greeting.