asterisk users - Apr 2005 - Auto Fax transfer problem?

I've been confused by the auto fax detect behavior. * reliably detects 
fax tones and initiates the xfer, but for some reason the destination 
extension usually disconnects immediately before ever ringing.

I can dial the fax extension directly (666) and the fax always 
rings/answers no problem. But with the auto xfer, seldom such luck. 
Sometimes it -does- successfully ring and answer on transfer, but only 
sometimes. The instant disconnect is the more frequent outcome.

Ideas anyone?

Here's what Asterisk log looks like on one of these failures.
 From Zap/1-1 I dial 345 so Asterisk answers and can hear tones. The two 
steps "Called 6" and "Hungup Zap/6-1" happen with near zero
delay so 6
doesn't ever get to ringing:

    -- Starting simple switch on 'Zap/1-1'
    -- Executing VoiceMail("Zap/1-1", "u222") in new stack
    -- Playing 'vm-theperson' (language 'en')
    -- Redirecting Zap/1-1 to fax extension
  == Spawn extension (andy-default, fax, 0) exited non-zero on 'Zap/1-1'
    -- Executing Goto("Zap/1-1", "andy-fax|s|1") in new
stack
    -- Goto (andy-fax,s,1)
    -- Executing Dial("Zap/1-1", "Zap/6|35") in new stack
    -- Called 6
    -- Hungup 'Zap/6-1'
  == Spawn extension (andy-fax, s, 1) exited non-zero on 'Zap/1-1'
    -- Hungup 'Zap/1-1'

Here are the relevant parts of extensions.conf

[andy-default]
exten => fax,1,Goto(andy-fax,s,1)
exten => 345,1,Voicemail(u222)

; Fax Modem
exten => 666,1,Dial(Zap/6)
exten => 666,102,Playtones(busy)
exten => 666,103,Busy

[andy-fax]
exten => s,1,Dial(Zap/6,35)
;exten => s,2,Hangup
exten => s,102,Answer
exten => s,103,Playback(carried-away-by-monkeys)
exten => s,104,Playback(lots-o-monkeys)
exten => s,105,Dial(Zap/1)
exten => t,1,Goto(s,102)