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.
TapeACall is one option if you’re looking to record the phone calls you make on your iPhone without having to create a new phone number. The app is only $11, making it a very reasonable investment for your business. In order to record a call on your iPhone with TapeACall, you basically create a conference call where one party is TapeACall.

Here’s a simple clinician voicemail script if you want to keep your message to the point. Hello. You’ve reached Highland Family Counseling Services. We are currently unable to take your call, so please state your name, phone number, and a detailed message after the beep. We will return all calls within two business days.
You do need a visual voicemail for the last option. That is something that is provided by your cell phone provider and most cell phone provider have it by default. if you don't see it, call them and see if you can add it.