Instead of paying for using your phone as a modem fee ($20 monthly fee!). You can use the following method get online and save money:

Make you you have vision or power vision service.
Download PDAnet from http://mytreo.net/downloads/modules.php?name=Downloads&d_op=search&query=pdanet, you must install the program and sync it to your Treo.
Then go to your treo and launch PDAnet and simply tap connect; now you're on the internet.

Using bluetooth dial up networking feature is tricky but do-able. Make sure you have the lastest driver for your bluetooth device.
I find that WIDCOMM has the best driver, I would suggest you to try it out!

Thank you!