This sounds something like Vonage to me - VOIP.

Describe your setup.
What type of internet connection do you have:
cable, dsl, dialup?

From the wall you have the internet connection to the modem.
What is the make and model of the modem?

You have a ethernet cable from the modem to your VOIP router.
From the VOIP router you probably have only two OUT ports: one for the
phone and one for the computer.


Now, it just occurred that you might be saying - you are wanting to use
a cell phone as a modem. If this is the case, I know Verizon works, AT&T
has some problems, and i dont know about spring with Ubuntu.