Another worth considering trick is to remove all the old voicemails that are consuming the storage and creating multiple dilemmas like this. To delete the voicemail here are the steps, Open the Phone app. Tap on Voicemail at the bottom tab. Then tap on Edit at the top-right screen. Select All the Voicemails and at last tap Delete.
Tap the Phone app to launch it. Tap Voicemail in the lower-right corner of the screen.Find the voicemail you want to delete. Tap Delete (or, on some versions of the iOS, the trash can icon) to remove the voicemail.

1. Select the fixed key with the envelope graphic, or dial the phone’s phone number.
2. Press and hold the "1" key on the keypad. If it’s your first time setting up voicemail, you’ll likely see a message that says “No voicemail number is stored on the card.” If pressing this button dials your voicemail service, listen to the prompts for instructions on completing the setup process.