asterisk users - Nov 2006 - Fast Busy with autodial using a call file

Setup overview:

We have an asterisk server serving a small number of SIP phones.  The 
asterisk server is connected to an old phone system via a T1.  The 
asterisk server is also connected to a second T1 used for 
inbound/outbound calls.

Scenario:

We are using a call file to do auto-dialing.  A call is made to a phone 
on the old phone system.  This call always works without a problem. 
When that call is picked up asterisk then calls a second number (either 
a SIP phone, another number on the old phone system, or an external 
number) and connects the 2 calls.  The second call is always dialed with 
a 91NXXNXXXXXX so that it goes out over the T1, even if it is destined 
for an internal SIP extension or an extension on the old phone system. 
If it is destined for 1 of those 2 locations it is wrapped back through 
the T1 and comes in as an incoming call.

If the second call is to either an extension on the old phone system or 
an external number then it works fine.  If it is destined for one of the 
SIP extensions it fails with a fast busy about 95% of the time.  The 5% 
of the time when it does work seems to be when I have not used a call 
file to call that extension in a long time (say 10 or 15 minutes).  So 
it seems that something may not be getting closed correctly.  Calling 
manually to the same number works 100% of the time.

Any ideas why we might be getting a fast busy when using a call file?


Related configs:

The call file looks like this (with the extensions altered):

Channel: Zap/g2/6070
RetryTime: 60
WaitTime: 10
Context: internal
Extension: 913125551212
Priority: 1

extensions.conf:

TRUNK=Zap/g3                ; Trunk interface
...
[internal]
...
exten => _91NXXNXXXXXX,1,Dial(${TRUNK}/${EXTEN:1})
exten => _91NXXNXXXXXX,n,Congestion()

zapata.conf:

context=internal
switchtype=national
signalling=pri_cpe
group=3
channel=> 6-27