I am trying to record voice anywhere. That is why I need a way to isolate the voice and reduce ambient sounds such as cars or trains. Ideally I want to do this with an iPhone, which does not have a high quality microphone.
You will need a portable sound-booth, a good pop-filter and record very close to the mic.
Commerical portable sound-booth:
There are a bunch of DIY sound-booths you can build yourself at a low price: