Are there any iPhone apps to record a phone conversation?

The police said if I could record a certain a phone call that it will help them a lot so this is very urgent any help is appreciated.

I thoughth the iPhone had one built in.
though there apps as well.

go to the appstore and search for "record"
https://itunes.apple.com/...41203?mt=8 - example

some discussion here
http://forums.macrumors.com/showthread.php?t=847349

and more on google for "apple store phone conversation recording"