I don't run ubuntu, but modem setup is plain vanilla. Posting now from old laptop running SuSe 10.0.
the modem device is ttyACM0 (or ttyACM1, depending on your configuration). You may need to modprobe cdc-acm to get these devices. I'm using *99***4# as the phone #. Insure that the apn is defined in the 'internet settings' on the phone, e.g., internet2.voicestream.com. Set baud rate of 115200. Here's modem init strings:
AT Q0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
User name and pasword are blank. Use open DNS. I use kinternet, which provides a convenient widget on the desktop for dialup/hangup. - hth