Because there is no built in method (due to privacy laws) your top solutions are Google Voice (incoming calls only) and the Voicemail Merge trick. Then you have apps. Apps are not my speciality so I'm sticking with the first two tricks. Links on each follow.
http://osxdaily.com/2016/11/02/record-iphone-calls-voicemail/ (voicemail merge)
https://www.digitaltrends.com/mobile/how-to-record-calls-iphone/ (google voice, incoming calls only.)
Both methods are zero dollars.
I am wondering how to record a phone call (e.g. like a phone interview) on iphone 6. Is there a built in feature? A good app?