On a Sansa MP3 Player and the supplied headphones so that it works without any problems.
For my iPhone it is unfortunately not at all suitable.
If I completely plug in and unplug the iPhone headphones into the socket of the extension cable, ever comes no sound.
I pull back half out the plug, I have sound. However, the plug is then only loosely in the socket - is not really a solution.
The difference in the headphone plugs is the following - the Sansa plug I see 2 rings, the iPhone connector 3 rings.
Conclusion: Sansa MP3 Player Headset OK, iPhone headphones total failure. Too bad, so only 3 points.