Win Server 2003 Dial-In Problems

I am having this really frustrating problem with Windows Server 2003. I have Remote Access installed and I have it setup to accept incoming calls from an external modem (a US Robotics Courier V.Everything 33.6 external modem). One of the users has a PDA with a 56k modem and is trying to dial-in to our server to access the internet. He can dial-in and the modem connected to the server picks up and connects. It then promptly disconnects about 1 second later. I've checked the security logs on the server and the oddest thing is happening. It says that the user successfully logs on, then the next item says something about successfully assigning special privledges, then the next line says the user successfully logged off. All three of these things happen within a second. Does anyone know why this would be happening? I have tried it with two different user accounts on two different devices (Both user accounts have their Dial-In property setup to 'Allow') and the result is the same. Successful logon then logoff within a second. Please Help!

Thanks

Although i would check RRAS, how about dialup from a standard PC? Then you would eliminate the PDA issue. Checked technet/windows 2003 communities?