[Freeswitch-users] NORMAL_TEMPORARY_FAILURE after 28 minutes of talking?

Hello, I'm getting frequent complaints from a customer about calls being dropped. It seems to be specific to this customer, as opposed to system wide. I found a call that they mentioned, and the call hangs up on the phone side (as opposed to the trunk side), with NORMAL_TEMPORARY_FAILURE after about 28 minutes of successful media. What would cause a call to cut off and return this cause code? Is there any way that loss of Internet connectivity to the endpoint would do this, or do we have to receive some kind of SIP message to generate that? Any help is appreciated. -Tim