I've always used a TRS cable plugged into line inputs and never had any trouble. That is like saying you have to have earphones with a mic/remote for them to work properly. Not the case. This is no different than plugging in a normal set of ear/headphones with a TRS connector. The phone knows how to handle it based on the connection it sees. A case on the device that would not allow the plug to seat fully into the jack is much more likely the issue than TRS versus TRRS.An iPhone uses a TRRS jack (4 conductor) to connect a mic and a volume control along with left and right audio. The audio cable you have is a TRS (3 conductor). That causes the phone to try to put some DC, possibly pulsed, onto the ground line. That causes noises in the audio. Buy a proper audio output cable that uses the "Apple connector" for the iPhone, and it'll work OK. Connect it to a line input, not a mic input.
This is a good idea but it will knock the signal down to mic level. If the mixer doesn't have preamps on the stereo channel, you will have to eat up two standard input channels.My solution to some of the issues with connection phones and computers to a sound system is this:
I believe the whirlwind PC DI has an attentuate switch which keeps the out put a line level signal, but continues to make it Balanced Stereo XLR.