They may have not re-assembled it correctly (Easiest way to test is make a phone call and place your hand over the top of the screen) the screen SHOULD switch off as they have a proximity sensor to shut the screen off when your on the phone to save power etc and to prevent inadvertant input from your ear etc, what happens when you do this?

If it does as I say then the connection is fine so you either have a faulty earpiece, (Easily sorted) or they can suffer another problem where the phone thinks it's got the headphones plugged in when it hasen't hence no earpiece!

Also there is no such thing as a WI-FI sim! It's the phone that has the wireless function, you can remove the sim if you wanted and still use it wirelessly! By doing this you effectivly have an Ipod!

Martin