"from CHANGES in the trunk:
------------------------------------------------------------------------------
--- Functionality changes from Asterisk 1.8 to Asterisk 10 -------------------
------------------------------------------------------------------------------
...
chan_dahdi
--------------------------
* Busy tone patterns featuring 2 silence and 2 tone lengths can now be used
with busydetect. usage example: busypattern=3D200,200,200,600"
So you'll need to upgrade to Asterisk 10 if you want to use that feature.
