Why is my voice mail full when it only has 18 messages?

Deleted voice messages remain

You have to scroll to the button> open deleted voice messages> Clear All

• iPhone 5 / 5c / 5s
• iPhone 6 / 6 Plus / 6s / 6s Plus
• iPhone SE
• iPhone 7 / 7 Plus
• iPhone 8 / 8 Plus
• iPhone X

1. From the Home screen, navigate: Phone icon > Voicemail .
2. Tap the message.
3. Tap Delete.
4. Tap Deleted Messages.
   May require scrolling to the bottom of the page.
5. Tap Clear All (upper-right).
   To restore a deleted voicemail, tap the message then tap Undelete.
6. Tap Clear All to confirm.

Set up Visual Voicemail on your iPhone - Apple Support

Apple - Support - iPhone - Voicemail Troubleshooting

Ask your cellular carrier. Voice mail is a carrier feature.