Most of the time, an update to your carrier’s voicemail app or settings will cure the issue; however, it is important to confirm that your voicemail number is appropriately configured before proceeding any further.
Note: Voicemail, Visual Voicemail, and Voicemail transcription are available from select carriers in select countries or regions.

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.
Update 23/9: Apple has updated its support document indicating that this issue can be resolved by installing the latest macOS Big Sur 11.6 update. In a newly published support document on its website, Apple has acknowledged an error that some users may receive when they try to use a scanner with a Mac in the Image Capture app, Preview app, or the Printers & Scanners section of System...