I was trying to connect a PDA (ASUS 696) to the Internet through a Zapp 810i phone (in Romania). I successfully created a dial up networking connection from the PDA to the phone using a Bluetooth Modem, configured to dial #777 and to use the appropriate Zapp username and password. But the connection, after initiated successful, was always disconnected with "unknown reason" as the error message.
The resolution was to restart the PDA (I shut down it and then restarted). Note that for ASUS A696 to restart the PDA you need to hold the power button pressed for about 4 seconds, then answe Yes at the notification asking if you want to shut down, then wait a few seconds for the PDA to stop and start it again.