asterisk users - Aug 2011 - How to stop Dial from waiting for extra digits if number is incomplete.

We have Asterisk 1.6.2.17.2 connected to a sangoma E1 card. The problem 
we are having is that we have a calling card type application and when 
people enter the number to be dialled we call the Dial application. It 
gets back an indication that the number is incomplete (via PRI cause 
code 28 I assume) and waits for extra digits. If it gets extra digits it 
appends them to the current extension and jumps to priority 1.

Is there any way this behaviour can be changed so if Dial tries dialling 
an incomplete number it just fails and we can query ${HANGUPCAUSE} to 
decide what to do.

Thanks
Gareth