asterisk users - Mar 2005 - Native Bridging drops call on release

Has anyone experienced a dropped call when bridging? I get an "OK, 
ready to transfer" from both channels, but when asterisk releases the 
call, it is dropped immediately by the upstream provider. I've tested 
against another provider and it works fine, and it also works fine 
across two different providers, including TO and FROM the one that's 
acting buggy.

Here's a diagram of the my QA testing, where A and B are two IAX2 VoIP 
service providers:

A > A = OK
A > B = OK
B > A = OK
B > B = FAIL


I want to rule out anything on my end first, in case the bug is being 
exposed by something sloppy I've done. Here's the log:


     -- Accepting AUTHENTICATED call from nnn.nnn.nnn.nnn:
        > requested format = ulaw,
        > requested prefs = (),
        > actual format = ulaw,
        > host prefs = (ulaw|alaw),
        > priority = mine
     -- Executing Goto("IAX2/PROVIDER-B@PROVIDER-B/5", 
"inbound-fax|s|1") in new stack
     -- Goto (inbound-fax,s,1)
     -- Executing Ringing("IAX2/PROVIDER-B@PROVIDER-B/5",
"") in new
stack
     -- Executing Wait("IAX2/PROVIDER-B@PROVIDER-B/5", "2")
in new stack
     -- Executing Answer("IAX2/PROVIDER-B@PROVIDER-B/5", "")
in new stack
     -- Executing Dial("IAX2/PROVIDER-B@PROVIDER-B/5", 
"IAX2/xxxxxxxx@PROVIDER-B/1202xxxxxxx") in new stack
     -- Called xxxxxxxx@PROVIDER-B/1202xxxxxxx
     -- Call accepted by nnn.nnn.nnn.nnn (format ulaw)
     -- Format for call is ulaw
     -- IAX2/PROVIDER-B/6 answered IAX2/PROVIDER-B@PROVIDER-B/5
     -- Attempting native bridge of IAX2/PROVIDER-B@PROVIDER-B/5 and 
IAX2/PROVIDER-B/6
     -- Channel 'IAX2/PROVIDER-B/6' ready to transfer
     -- Channel 'IAX2/PROVIDER-B@PROVIDER-B/5' ready to transfer
     -- Releasing IAX2/PROVIDER-B@PROVIDER-B/5 and IAX2/PROVIDER-B/6
     -- Hungup 'IAX2/PROVIDER-B/6'
   == Spawn extension (inbound-fax, s, 4) exited non-zero on 
'IAX2/PROVIDER-B@PROVIDER-B/5'
     -- Hungup 'IAX2/PROVIDER-B@PROVIDER-B/5'