You can’t always get to the phone when clients, partners, investors, or colleagues call you. When this happens, a professional voicemail greeting helps you make a positive impression and tells them what to do next. There are a variety of ways to convey your message. If you’re not sure what type of message to record for your own voicemail box, here’s a definitive guide to get you started.
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.

You would think there'd be app for that, but Apple doesn't allow third-party apps to access the microphone and the integrated Phone app directly. However, there are tricks within some apps and services to get around that limitation that can make you a mobile wire-tapping master.
What kind of company do you run? A law firm may prefer more formal business voicemail greetings than a trendy retail store, for example.