There is no definitive answer here, but you should include one of the two at the beginning of your message based on what makes the most sense.
My iPhone rings for 15 seconds – which is actually eight rings – before it goes to voicemail, but even that’s not long enough if I’ve put the phone down in another room or it’s zipped into a pocket in my jacket.

Open your Phone app. Tap Voicemail. Select Greeting. By default, your voicemail will be setup with your carrier's generic greeting. To record a custom greeting, select Custom. Tap Record to begin recording your voicemail greeting. When you finish recording, tap Stop. To make sure you're happy with your new greeting, tap Play.
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.