asterisk users - Apr 2010 - Busy(20) returns non-zero and exits immediately on IAX channel

Hi,
I'm running Asterisk 1.4.26.3 and I've noticed an interesting problem
when trying to play a Busy tone over a IAX trunk from the PSTN.
It seems as though Busy(20) returns non-zero immediately (it does not
wait 20s), so the caller never hears the busy tone, but
the call just appears to hang up.
I don't believe this happens when trying to play a Busy on a SIP trunk.

The busy part of the dialplan looks like this,

exten => s-BUSY,1,Noop(Dial failed due to trunk reporting BUSY - giving up)
exten => s-BUSY,n,Playtones(busy)
exten => s-BUSY,n,Busy(20)

The only way to remedy this is to put a Wait(20) between the
Playtones() and Busy().
Any ideas on why this only fails on IAX and not SIP?

Thank you.

-- James