If you use voice mail or an answering machine to answer calls when you can't, make sure that you have a professional message recorded, that does the same thing as tip #3, and gives callers any other pertinent information before it records their messages. Update your voice mail message as needed.
Select the Phone app and select Voicemail.If you see a message that says Set Up Now in the bottom corner of the voicemail screen, you have not set it up.Tap Set Up Now and follow the wizard. You will need to set up a greeting and a password to get it working.Check signal strength

Most carriers offer voicemail for free. However, some carriers may limit the number of voicemail messages you can have stored. But this shouldn’t be a problem because not many callers leave voicemails anymore.
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.