asterisk users - Aug 2005 - Dialplan to dial SIP, but stop dial on analog pick up?

Most of the documentation I have read through shows dial plan examples that
dial the SIP phones and stop if one is picked up. I have not seen an example
of or read how to stop the SIP dial when an analog phone is answered.

How can the extension be set up so that when an analog phone is picked up
the SIP dial stops?

extensions.conf
_________________
[incoming]
exten => s,1,Dial(SIP/2001&SIP/2002,20,tr)
exten => s,2,Answer
exten => s,3,Hangup

[outgoing]
exten => _9X.,1,NoOp("Call for "${EXTEN:1})
exten => _9X.,2,Dial(Zap/1/${EXTEN:1})

[default]
; desktop
exten => 2001,1,Dial(SIP/2001,30)
exten => 2001,2,Hangup
exten => desktop,1,goto(2001,1) ; To be able to dial with text,
"sonya"

; Laptop
exten => 2002,1,Dial(SIP/2002,30)
exten => 2002,2,Hangup
exten => laptop,1,goto(2002,1) ; To be able to dial with text,
"laptop"

include => outgoing