Some mobile networks automatically set up a default PIN for your voicemail system, which may be something as simple as ‘1234’. That’s really easy to guess, so if someone knows your mobile phone number then they may be able to get into your voicemail just by guessing your PIN.
Set up Visual Voicemail. If your carrier supports Visual Voicemail, you can set it up with these steps: 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.

Once the voicemail is done, click the "Stop" button at the bottom part of the tool. You can find the recorded audio by clicking the three parallel bars at the right bottom of the recorder. From there, select the recording track and click the "Play" button to listen to the recorded voicemail. You can also click the "Folder" icon at the rightmost part of the Recording List section to open its output folder.
The simplest, lowest-tech option—beyond operating a recorder while you blather over the speakerphone—is a cable: the Olympus TP-8 Telephone Pick-up Microphone for under $20. It doesn't digitally capture from your iPhone. Instead, it has a microphone built into the earpiece. Plug the 3.5mm plug on the other end into a recorder. Hold the iPhone up to your ear to talk normally. The TP-8 captures each side of the conversation from what comes out of the iPhone's ear speaker, while you can still hear the conversation.