Voice transmission breaks

I just installed a new AsteriskNOW (Asterisk version 1.4.32) server with freepbx (version latest 2.7). My problem is that when I make a call to a featurecode or any other extension, I hear voice, but 5 or 10 seconds later I don’t hear sound anymore. I have enabled NAT and configured it with the correct addresses, I also configured my RTP ports. What could be cousing this problem? Thanks!