asterisk users - May 2006 - Reasons for a SIP channel to hang ?

Hi all,

Simple question;

What are the possible reasons for a SIP channel to hang there for hours 
after
the call has terminated ?

Calls are sent from 1.2.6 to SER using DeadAGI over the internet, and 
yesterday there
were 100 calls that hanged for hours over a total of 15k calls.

Typical day would be 3 to 5 calls, I also noticed that a reboot helps to 
resolve this issue.

Thanks,
Fred