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Let us begin with the most obvious. Have you set up visual voicemail on your iPhone?

Alright let me try to clear this problem as much as I can. From the parent carrier, we have a limitation or you can say a requirement that all numbers should have the Voicemail set up. So what we do for those customers who don’t want it, we disable it from our side but again the numbers keep syncing from the parent carrier’s network from time to time (roughly once a quarter). When the numbers get synced, voicemail automatically gets enabled. To some people, this is alright but sometimes it becomes inconvenience for the customers to call us again to disable it.
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.