asterisk users - Jul 2007 - Keep playing Background while dialling invalid dtmf extensions

hi asterisk users

How can i make asterisk "ignore" invalid extensions, and go on playing
the
background soundfile?

Normally, asteriks will take the user to the invalid extension if the caller
presses anything other than 1 or 2 in the following context::

  [example]
  exten => s,1,Answer()
  exten => s,2,Background(hello-world)
  exten => s,n,Goto(s,2)
  exten => i,1,Playback(invalid)
  exten => i,n,Goto(s,2)
  exten => 1,1,Goto(sub1,s,2)
  exten => 2,1,Goto(sub2,s,2)


I'd like asterisk to goto `sub1` if `1` is pressed or to `sub2` if `2` is
pressed.
However, if a users presses `3` asterisk shall go on playing `hello-world`
and wait
for a correct extension.

If i leave out the i extension the call gets terminated when dialling a
wrong extension.


Any ideas or pointers welcome.

regards, fRiSi

ps:
yesterday i wrote the same email to this list from another email account.
as it did not appear in my newsreader i'm writing this second one.
if the first one will arrive too, sorry for double posting.