It is possible to connect the EK 100 G3 receiver to an iPhone that has a headset jack socket for recording audio from the wireless microphone.
To do this requires two cables. First you will need the Sennheiser PCV07 adaptor for your iPhone. This converts the single 4 pole jack, into two 3 pole jacks - one for the microphone and one for headphones (for monitoring).
The second cable needs to have a 3 pole TRS jack (Tip-Ring-Sleeve) at each end and must be wired as follows:
Tip of jack 1 to tip of jack 2
Sleeve of jack 1 to sleeve of jack 2
Rings of both jacks not connected