No Voice coming in

I setup a extension for my friend, he lives in another city, to help me test the Asterisk box. When the connection established between two extensions, he can hear me but the is no voice come from his side.

We both use X-lite softphone and we both behind router. So is this a NAT problem? How can I fix it?

You must explicitely set
(in sip.conf, or in approptiate MySQL table)
We also had simmilar problem but with Sjphone under Windows.
After restart of windows - no more problems.