Recording calls on an iPhone is not that hard. There are a bunch of apps that let you do that on the go. The Photofast Call Recorder X For iPhone takes it to the next level though. It lets you start and stop recording at the push of a button. Audio is recorded to a microSD card, so you won’t have to give up any space on your phone.
The Call Recorder X weighs only 11g. It comes with an app that lets you manage recordings and even have them transcribed.