asterisk users - Mar 2005 - Outgoing call immediately disconnected

I have created a call file as shown in the files below. The number is 
dialled and
connected (i.e. the call is placed to the PSTN) but it is immediately
disconnected and I get the following message on the console:
Starting Zap/3-1 at from-internal-custom,s,1 failed so falling back to exten
's'

Extensions.conf
[from-internal-custom]
exten => s,1,Wait(20)
exten => s,2,SendDTMF(1)
etc

1.call
Channel: Zap/3/1234567
MaxRetries: 1
RetryTime: 60
WaitTime: 30
Context: from-internal-custom
Extension: s
Priority: 1

Does anyone have any suggestions as to why Asterisk is failing when it tries
to access this extension in this context?

Regards

Cameron

P.S. I tried sacrificing chickens but it didn't work