asterisk users - Nov 2006 - no sound when bridging 2 asterisk SIP connections

Hello, 

here is our layout:

	asterisk-A <--- WAN ---> asterisk-HQ <--- WAN ---> asterisk-B

calls are routed with SIP between asterisk's (found IAX to unreliable). 
When asterisk-HQ attempts to native-bridge OR simply forward calls 
between A and B no sound is sent.

If either leg (A <-> HQ or B <-> HQ) is converted to IAX, then sound
flows normally.

We are using 1.2.13.

What could be the problem?