Q: I have a little guitar interface box that I connect to my iPhone for practicing. It hooks up to the iPhone headphone jack with a 1/8″ cable. It was working fine, but now I can’t get the guitar signal to show up inside my iPhone. What happened?
A: We’re betting that you switched cables and inadvertently are using the wrong type. For audio interfaces that connect to the headphone output/mic input jack on an iPad, iPhone, or iPod touch, you need a two-way 1/8-inch TRRS (as opposed to a TRS) cable that both sends signal into the mic side and then routes it back out of the headphone side.
It’s very easy to accidentally grab a 1/8″ TRS cable when what you need is a 1/8″ TRRS cable — the two types look very similar. Double-check the cable you are using, if you have the wrong type, swapping back to the right one should fix things!