Is an iPhone call recorder theoretically possible?

On Jailbroken iPhones, SpoofApp does call recording. I am not a laywer, but note that recording calls may not be legal depending on where you are and/or where the other party is.

For the UK market there is also Call Recorder . It uses a premium rate number to record the call but it's available for non jailbroken iphones too.

Just recently I discovered an app actually able to do it properly, but just for SIP calls.

It's called MobileVOIP and it's on Apple Store! :D

So I hope this does answer your question. Even if it's not recording a phone call, I believe it shows it's theoretically possible.

There is no easy way since that part of the software and hardware is secured.

Of course it's possible in the theoretical sense as the voice recorder and music mixing apps show the power of signal processing hardware and software has access to both streams of sound.

Someone would need to re-do some or all of the iOS code and change it. But what's simple in concept is surely complex and perhaps too costly to happen in reality.

You simply take the code that listens to the stream of microphone data inside the mobile Phone app and dump a file to storage as well as encoding it for voice calling. If that app calls an API rather than doing that processing itself, then you would implement the recording there. You would do the same for the incoming data and could either add time markers for later mixing or mix it down as the call is being handled.

This code is of course not in place using the documented API so you would need to patch the app after a jailbreak or uncover an undocumented API to actually implement this feature.

The use case is so obvious that if this were sitting there, hundreds of apps would be doing this already.