How can I hear what I'm playing on the guitar while recording with GarageBand on my iPhone?

First of all: English is not my first language, but I figured I might get most answers on the English community, so I'm posting my problem right here, even though I might not now all the right words. So please bare with me while I'm trying to do my best 😉

So, I'm plugging my guitar directly into my iPhone 4 and record with GarageBand, but obviously since a cable is plugged into headphones jack I can't hear what the guitar sounds like using the selected amp (I especially tried using the Mesa Boogie styled amp). I turned on the monitor option, but this wouldn't help either. I can hear everything as soon as I plug the guitar out of the iPhone and play the recorded track, but with it plugged in I won't hear the metronome or anything for that matter.

But I figured there had to be a way, right? Cause otherwise this wouldn't make too much sense to me, meaning that I would've wasted some money on this app.

I hope anyone has any idea at all. Probably I'm just being too stupid to see an easy way around this... 😕

Thanks in advance

Sabrina