To set up voicemail on your iPhone: 1. First, tap Phone. 2. From the menu that appears at the bottom, select Voicemail. 3. On the next screen, tap Set Up Voicemail. 4. Enter a password you want to use for your voicemail, then tap Done. 5. Select a custom voicemail greeting or the default one.
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.

Tap Record a new greeting. Note: If necessary, delete an existing greeting (limit of 2 greetings) to make room for the new greeting: Tap the Menu key, tap Delete greetings, tap the check box next to the desired greeting, and then tap Delete.
Make sure that you set up your voicemail. To check, dial your own phone number from your iPhone and leave yourself a message. Call your iPhone from another phone and try to leave yourself a voicemail. Go to the keypad and press and hold on the 1, or press the Call Voicemail button. This dials voicemail. Check if you have any missed messages.