line-level audio into an iPhone headset jack?
Line levels are 0.5-2.0V range.  Mic inputs are expecting millivolts.  You just need what is called an L-pad attenuator.  Your drop should be 40-50 dB.  

If you are using a consumer line level, you will need about 40 dB attenuation.  (Professional audio line input would need about 50 dB.)



Attenuation in dB = 20 * log10((R1 + R2) / R2)

Try R1 = 10k and R2 = 100.

If you get accurate measurements of your line source and a know good mic signal for the iPhone, you will be able to calculate more accurate values.  Or you can put a 500 ohm trimmer resistor in place of R2 to allow varying of the signal if the line out doesn't have adjustment.