Is it possible to record phone calls on the iPhone? If so, how?

I've been getting threatening phone calls from a private number and since I can't trace the private number, I've decided to try and record the phone calls. Is there any apps, or a way, or even suggesting how I could possibly do it? I was thinking of putting it on loud speaker and recording it with a cassette or something.

Http://www.iphone5contest.info

Not only one, much:

http://www.brighthub.com/mobile/iphone/articles/82001.aspx

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