Just give someone a call on your iPhone.
After the call establishes, press the "Record" button located in the upper left corner of the call screen.
Both parties will receive a prompt saying, "This call will be recorded."
The notification cannot be ignored. The recording will end automatically if you do that.
To end the recording, press the "Stop" button.
Locate the "Call Recordings" folder by opening the Notes app.
Go to Notes > Call Recordings to access the recording.