If it is the first time you are using the Voicemail, you should set up or change your default voicemail on iPhone 7 first. If you have set up the voicemail, you can skip to next part. Or else, you can get some helpful information to set up voicemail. As different carriers may have different ways to set up voicemail on iPhone, the part just take AT&T for an example.
Rumored design changes include shorter stems like current AirPods Pro, but without advanced features like active noise cancellation.

iPhoneAlley details a technique to trick the iPhone into using any custom audio file as your voicemail greeting.
Go to menu, and click on messages. At the bottom of the screen, it should say options. Click on options. You should see a list of options come up on the screen. Click on the one that says voicemail. When you click on voicemail, it should call the automated voicemail. You should hear an automated voice giving you options. When you hear personal options, press the number that the automated voice said was for personal options. The automated voice should then list some personal options. When you hear `greetings,` press the number that the automated voice said was for greetings. The automated voice should list some greetings. When you hear `personal greetings,` press the number that the voice said was for personal greetings. Follow the automated directions from there.