As Dave says, the iphone shoots with a variable frame rate that FCP and others convert to a fixed frame rate but there will be slippage with a separately recorded track.
There are special movie apps you can load into your iphone for more control over video shooting that may produce fixed frame rate. I use one called MoviePro, though haven’t checked out the frame rate effects lately. That might be a solution.
If the iphone is recording sound with picture, I suppose one could import the sound as well as the picture into your NLE and use one of those programs that syncs your separate audio recording to the camera audio track. But this might produce some unwelcome sound anomalies of pitch, tempo, etc.
I think a special iphone movie app will probably be the solution. Needs researching and tests.
Cheers,
Robert Withers
Independent/personal/avant-garde cinema, New York City