iPhones uses a much narrower plug, so most headphones and cassette adapters won't work. I've cut away the plastic on my Etymotic 4 and 6is and this seems to work, but when I do it on my Sony cassette adapter, it causes the whole thing to break. A non-intrusive fix is to buy a dongle adapter. Etymotic told me about two:
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Wow you actually cut the plastic on the ER-4? Was afraid to do that since it's such a small piece. Personally I'm waiting for Shure's Music Phone adapter.