asterisk users - Nov 2006 - Wait for an extension and dial. Why does this not work?

>From my extensions.conf:
exten => 888,1,Answer()
exten => 888,n,WaitExten(20|m)
exten => 888,n,Dial(SIP/${EXTEN}@${SERADDRESS},60,tr)


This should:
* answer
* wait 20 seconds for an extension with music on the background
* pass the call to that extension on ${SERADDRESS}


What am I doing wrong here? I don't even get the background music while
WaitExten is active. I doubt that it is active anyway, since I get
disconnected before the 20 seconds have passed...  :-/


Greetings,
  Evert