When I speak into the phone, person on other end of call can't hear me

I’ve inserted my original sim card into a replacement Nokia 6555b phone. The numbers all come up and I’m able to send and receive texts. I can place a call to another person and I can hear that person very well; however, the person on the other end of the line cannot hear me even though I’m speaking into the phone. Volume is set to 10 (max) and still other person cannot hear me. What’s the problem and can it be easily repaired? Everything else about the phone appears to work properly.