There is a way to connect a cell phone to the internal modem, it's just really hard to find the gizmo. Here it is, I sold it already:
Motorola made something called The Cellular Connection. It is a box that connects to a Motorola flip-phone and has an RJ-11 connector on it which makes it super-easy to connect to any laptop modem, fax machine, credit card swiper, etc. They don't make this gizmo anymore and it was too bulky for me to carry around but it did work. It looks like this (note that I sold this already and don't know where to get more)
http://www.hawaiistore.com/ebay/cellularconnection.html
There was another company that made a similar product and you could also get an RJ11 jack on some of the car booster packs for the flip phones.