Start by holding the iPhone like this. You will notice that the speakers have been amplified immediately, and the sound will get slightly deeper.
Step 2: Cup
Cup your left hand around the iPhone and right hand like in the picture. This boosts the sound quite a lot, and the sound gets much deeper. Make sure that there are no spaces between your fingers, as this will ruin the tone.
Step 3: Adjust
Adjust your hands and experiment to get the best tone. If someone has a variation of this technique that works better, feel free to put them in the comments!