[asterisk-users] How to stop ringing when incoming PSTN call is answered externally?

[ftarz at mindspring.com]

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