Please check to see if you can access the Internet with your phone by itself. If this works then we know that some other issues are involved. Check to see if you phone pairs correctly with your Nokia 770 ( this can be done if you are able to explore the files on the phone via the Nokia 770 ). Check if your Internet provider still allows Bluetooth Phone tethering. I had an issue wherein for sometimes I could not get to the Internet using the normal settings used to use the phone as a Bluetooth Modem. I had to change some configurations to get this to work again, and that how it stays for at present.
I do not how much you Phone Service provider will be willing to help you.