asterisk users - Oct 2009 - Busy() returns immediately on IAX trunk

Hi,
I have two asterisk boxes A&B connected together via IAX.
Phones register to Asterisk box A, and Asterisk box B is the PSTN connection.
When dialing a number from a phone registered to A that DAHDI returns as BUSY,
the Busy(20) application returns immediately instead of playing the busy tone.
The user, instead of hearing the busy tone, gets an immediate hangup.

Why is this? Busy(20) seems to work fine if there is a SIP trunk
between the boxes,
but for other reasons I cannot use a SIP trunk.

Thank you.

-- James