The audio output of the iPhone headphone jack is quite likely at a high enough level to overdrive the microphone input causing clipping. Here's a thread regarding the output level:
I suspect that the level isn't high enough to cause damage to the microphone input, but I don't accept any responsibility if you damage a phone. That said, you should be able to just wire it up and determine whether the resulting recording quality is satisfactory.
If the audio quality is not satisfactory, which is what I expect, you will need an attenuator to reduce the level enough to avoid clipping. There exist audio cables with built-in attenuation, which I'd used with other equipment for feeding line-level output to microphone-level input. Here's an example:
I have not actually purchased that specific product, so I can't attest to its quality or suitability. The 25dB attenuation it provided could be too much. Also you will still need an adapter for the microphone plug end to connect it to the appropriate microphone and ground contacts of the iPhone TRS headphone connector.
You could also wire up your own T-pad attenuator for whatever amount of attenuation you need, made of three resistors. For instance, if you use three 20 ohm resistors, with two in series between the source and load, and one as a shunt between the middle of the series pair and ground, you'd get an attenuation of about 11.5 dB, and an impedance of about 34.4 ohms. There are various online calculators for T-pad attenuators.