With this method, you don’t need a third-party app. Call the person you want to chat with, then tap the “Add Call” button on your phone’s screen. Dial your own phone number—the one you’re calling from. This will take you to your voicemail. Once your voicemail has started recording, tap the “Merge Calls” button on your screen. Now, you’ll record your phone conversation onto a voicemail message. When you’re done with the call, you can tap the share button to export it to your Mac via Airdrop, to the Voice Memos app, or transfer it via email or text message. The file is recorded as an M4A, so depending on what you want to do with it, you may need to convert it to MP3 before uploading it to other apps.
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You can delete individual voicemails as you listen to them, and most networks will automatically delete old voicemails after 30 days. However, few networks offer a simple way of deleting multiple voicemail messages when your mailbox is full – unless you can use an option such as Visual Voicemail, which lets you delete messages with a quick tap of your finger.
Website: https://www.digitaltrends.com/mobile/how-to-set-up-voicemail-on-an-iphone/