Interface between IPhone / IPod and RTX can made by a standard Digital Mode Interface.
You can buy one on e-bay or make it by youself.
Genarally, these interfaces come with 2 stereo jack, one for audio in (mic) and one for audio out (speaker).
Last step, you must wire the 2 stereo jack in one 4 pin jack. Only for reference, the picture above show you the pinout of an IPhone.