1. I can't hear anything
Many people have had a issue where they cant hear anything from the wholesale iPhone 4 MC603B/A. The best solution that has always worked is by resetting the iPhone by going to Settings > General > Reset (at the bottom) > Reset all settings.

Its normal for the iPhone to turn off then back on in this step. Note you will lose all settings including icon arrangements but this will NOT lock your iPhone.

2. My iPhone has froze? What can i do?
If a application has froze holding down the home button for about 6 seconds should bring you back to the menu of the iPhone.

Otherwise the next best step is to reset your factory unlocked iPhone 4 (http://www.apple2010.com/Discount-iphone_c2) MC603B/A by holding the power button and the home button at the same time. When the device turns back in release both.