iPhone:Go to the Phone app, then tap the Voicemail tab. Tap Set Up Now.Create a voicemail password, then tap Done.Enter your password again to confirm it, then tap Done. Select Custom or Default. If you choose Custom, you can record a new greeting.Tap Done to save your greeting.Android:From a Home screen navigate to Phone icon > Menu icon > Settings.
This can also work with your iPhone or Android, if the above methods aren’t working for you. Just connect your phone into the computer with a 3.5mm audio cable. More on that in a minute.

After pressing the Voicemail tab, all of the voicemails stored on your device will display. From here, you’re going to want to tapGreeting located in the top left. There will be two options located on the Greeting page: One option to utilize the default voicemail message and another to record a custom message.
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.