1. Open your Phone app and tap "Voicemail" in the bottom-right corner. Here, you can delete voicemails indiviudally by swiping left on them and tapping "Delete."
2. If you want to delete all the voicemails at once, tap the word "Edit" in the top-right corner.
3. Tap on the circles beside all the voicemail in your inbox that you want to delete, then hit "Delete."