asterisk users - Nov 2006 - asterisk sip doesn't see other asterisk-sip

Hello,

Here is our setup:

asterisk-A <--LAN--> nat-router <--Internet--> asterisk-B

A and B have appropriate friend entries in their sip.conf with a 
qualify=yes.

The router forwards anything on sip,iax and sip/rtp ports to A.

The problem: SIP/A remains UNREACHABLE for SIP/B, however A sees B. No 
problem with iax2.

What did I miss in my configuration?

Thanks,