Look the kernel log (type dmesg in a console) just after plugging your phone.
If you can see something like:
usb 2-3: new full speed USB device using ohci_hcd and address 15
usb 2-3: configuration #2 chosen from 1 choice
But nothing else, you're probably missing the kernel USB ACM driver.
In 2.6 kernels it's called cdc-acm, and you can find it in
Device Drivers->USB support->USB Modem (CDC-ACM)