Motorola V710

I just bought my phone from ebay and it looks like it's new had screen protective plastics and what not but i think it may be refurbished...well the problem is that when i make a call i can hear the person on the other end but they can't hear me... i can hear myself in my own phone like the mic is feeding back to my speakers or something...if i use a headset the other person can hear me...i would appreciate it someone knew a quick fix for this problem cause i really don't want to send it back and wait for a new one..i was thinking could the mic be backwards or something? if so how do you take this phone apart, i really don't want to leave any noticable marks on it just in case i have to return it. Thanks.