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.
And then select your language preference. Enter the name and greeting information before you tap on "Set Up Now" option in the middle of the page. Just follow the similar method to change the default voicemail on iPhone for the other carriers.

FROM YOUR WIRELESS: Press and hold the 1 key to access the voicemail system. When you hear the recorded message, press the # key to bring you to the system menu. Press 4, for the Personal Options followed by 2 for Personal Greeting. Follow the prompts to record, listen to, re-record and save your greeting.
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.