asterisk users - May 2005 - how to detect a hang-up in the first 5 seconds

Hi,

how can i detect a call which hang-up in the first 5 seconds.

when i dial using sipgate.de without any money on my account .... i get an 
answer from sipgate and a hang-up after 3-5 seconds. 
So if they do so, i would like to choose the next provider.

Any ideas??

bye