Server 2003 Dial-In Problems ... Please Help!

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 Privileges, 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,
Bob