asterisk users - Jan 2010 - StopPlayTones() after first digit?

I configured our SIP gateway to automatically dial extension "s" when
a
phone is picked up. I want Asterisk to play a dial tone, wait for an
extension to be dialled, and hangup on timeout

This works great, but I also want Asterisk to *stop* playing the dial
tone after the first digit is pressed

So far my extensions.conf contains,

[internal]

exten => s,1,Answer
exten => s,n,PlayTones(dial)
exten => s,n,WaitExten

How can I continue playing a dial tone as long as a digit isn't pressed
(and TIMEOUT() hasn't expired) - but stop playing the dial tone as soon
as the first digit is pressed?

I think the Background() application works like this - it stops playing
as soon as the first digit is pressed - it seems PlayTones() works
differently?