I am not sure if it works with your system. Here is the setting I am using to connect to the internet (same service I am using with home telephone line) via BT phone (modem) dialup. My device: iPAQ 5550 and Ti68.

Assuming that your BT phone is BT ready and discoverable (I have ommitted "Next")

Start/Settings/Connections tab
Connections/Bluetooth Settings
Add a new modem connection
Enter name for the connection: Any name you like
Select a modem: select Bluetooth Dial-up Modem
Enter the number exactly as it should be dialed ...: Dial-up number provided by your ISP
(Use dialing rules is optinal depending on if you need to change the your location frequently)

User name: as provided by ISP
Password: same PW you have used with fixed-line dial-up
Domain: Depends on ISP (for my case I have ignored)
Advanced: It could be ignored for my case
Finish

Then in the Bluetooth Manager:
New/Connect
Select Connect to the Internet
Select Connect via dialup device such as cellular phone...
Select the device by tappin on <No device selected> . Select your paired BT phone
Select Create a shortcut for this connection
Rename Connectuion name if you want
Finish

In the Blutooth Manager/My Shortcuts
Tap on the shurtcut for the dialup you have created
Bluetooth Dial-up Connection/Select the connection to use: Tap on the connection you have created
You should be able to see the dialog box: Dialing:(dialup number)
If the connection is O.K. you can see it by tapping on the Active Connections tab.

You may access the internet now.

Hope it works with you system also.