Recording your personal calls is a massive legal gray area, since it's technically illegal to record a phone call when all parties aren't fully aware. So that's most likely why Apple doesn't include the feature baked into iOS.
How to record a call using Google Voice
Within the Google Voice app is a switch that lets you record incoming calls. In fact, that switch also enables other options like conference calls. Here's how to do it.
Download Google Voice from the App Store.
Launch Google Voice. You'll have to set up an account and choose a phone number based on your location.
Tap the three dots on the left side of the screen.
Tap the switch next to Incoming call options.
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