asterisk users - May 2012 - How to stop ringing when incoming PSTN call is answered externally?

This is a hard one to explain.  My home PSTN line is connected via an
Openvox A400P card to my Asterisk 1.6.2.23 box which then routes
incoming calls to my 2 SCCP extensions.

The calls are routed just fine, but when a call is answered at one of
the extensions or externally (by a home telephone) the asterisk
extensions continue to ring one more time.  Is there a way to have
Asterisk drop an incoming PSTN call as soon as it's answered?

CLI output when receiving a PSTN call:
   Starting simple switch on 'DAHDI/3-1'
      -- Executing [s at from-pstn-3:1] Wait("DAHDI/3-1",
"1") in new stack
      -- Executing [s at from-pstn-3:2] Verbose("DAHDI/3-1",
"CALLERID is
XXXXXXXXXX") in new stack
CALLERID is XXXXXXXXXX
      -- Executing [s at from-pstn-3:3] Verbose("DAHDI/3-1",
"Time is
20120327-204307") in new stack
Time is 20120327-204307
      -- Executing [s at from-pstn-3:4] Dial("DAHDI/3-1",
"SCCP/1000&SCCP/1100,30") in new stack
      -- Called 1000
      -- Called 1100
      -- SCCP/1000-00000038 is ringing
      -- SCCP/1100-00000039 is ringing
    == Spawn extension (from-pstn-3, s, 4) exited non-zero on
'DAHDI/3-1'
      -- Hungup 'DAHDI/3-1'

[from-pstn-3]
exten => s,1,Wait(1)
exten => s,n,Verbose(CALLERID is ${CALLERID(num)})
exten => s,n,Verbose(Time is ${STRFTIME(${EPOCH},,%Y%m%d-%H%M%S)})
;exten => s,n,Answer
exten => s,n,Dial(SCCP/1000&SCCP/1100,30)
exten => s,n,Hangup

On Tue, May 22, 2012 at 11:32:19PM -0400, ftarz at mindspring.com
wrote:
>The calls are routed just fine, but when a call is answered at one of
>the extensions or externally (by a home telephone) the asterisk
>extensions continue to ring one more time.  Is there a way to have
>Asterisk drop an incoming PSTN call as soon as it's answered?
I have the same problem, and earlier discussion here suggests it's
insoluble.