I believe you will need a 4-conductor TRRS connector to tap into the iPhone's mic input (using the Lightning to 1/8" adapter). Like what you see on earbuds that have a mic built in.
If you are just using an 1/8" TRS (into the Lighting to 1/8" adapter), you are only getting the L/R headphone outputs from the iPhone. You need to access the iPhone's input on that extra "Ring" connection (TRRS). This input is likely looking for low-level mic signals, so you'l likely need to run the FF800 output very low to avoid over-driving the mic stage...
Some of RME's USB interfaces can actually work as a USB audio device for iOS, and would likely be a better solution. But making what you already have work is always rewarding
MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8