To use UMTS Internet via a bluetooth mobile phone, install the packages gnome-ppp and ppp.

There’s a nice manual at UbuntuUsers.

1. use hcitool scan to find the MAC of your mobile
2. use sdptool search DUN to find the dial-up networking details of your mobile, note the line Channel: 7

3. edit the file /etc/bluetooth/rfcomm.conf and uncomment the section and make the changes according to your details:

    rfcomm0 {
    	# Automatically bind the device at startup
    	bind yes;
       
    	# Bluetooth address of the device
    	device 11:22:33:44:55:66;
       
    	# RFCOMM channel for the connection
    	channel	7;
       
    	# Description of the connection
    	comment "SonyEricsson P1i";
    }
   

4. launch Gnome-PPP
5. click Setup and change the settings:
   • Modem Tab
     • Device: /dev/rfcomm0
     • Type: USB-Modem
     • Speed: 460800
     • Wait for dialtone
     • Init Strings
       • Init 2: ATE1 (ATZ is sent automatically by Gnome-PPP)
   • Network Tab
     • Leave everything as it is
   • Options Tab
     • Ignore terminal strings (stupid mode)
6. enter following details for dialup:
   • Username: t-mobile
   • Password: tm
   • Phone number: *99#