I'm trying to play an audio file from my Mac Mini over an iPhone call. I tried using a male to male audio cable to connect the Mac's audio out port, into the iPhone's headset jack, but I guess that would be too easy. I also have a headset splitter but that doesn't seem to work either. Any suggestions would be appreciated.
You should be able to do something like this with a product like IK Multimedia's iRig (or a generic equivalent... of which there are plenty).
The iRig splits the shared audio input and output on the iPhone/iPad headphone port. Plug a patch cable from your Mac's headphone port to the input on the iRig (you might need adapters to go from mini headphone to standard... 1/8" to 1/4").