asterisk users - Nov 2005 - Avaya 4620SW Invalid Subscription-State - Issue

Hello All,
I'm using an Avaya 4620SW with Asterisk, the phone when hooked up to the
network, works for sometime, (I have not actually monitored the time) maybe
20-30 minutes, after which the phone will still have a dial tone, but can't
dial out or recieve calls. I scanned thru the logs and found this:
Got SIP response 400 "Invalid Subscription-State" back from
192.168.202.200(
192.168.202.200 is the static IP assigned to the Avaya device).

Thanks
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