Well, yes and no.
There is the limitattion of the cell phone. Do you have a digital or analog cell? If you have an analog cell, it should be possible to use the phone as the transmission medium. However, digital cell phones will not work. Digital cell phones samples the sounds only at the frequencies within your hearing range. This is used to save bandwith as well as to decrease packet sizes. There is a special method in which digital cell phones ferry data back and forth- which uses a special base station and a "seperate line" of sorts- like a DSL type connection to provide data services over the network. That is why in order to use your digital cell phone as a modem, not only do you have to buy a special connecter kit, but also a service which provides the back end services to enable data transmissions.
I have never owned an analog cell phone, so I could not tell you much about it. However, the little I know of the way it functions, it may be used as a transmission medium assuming it sends out the whole sound frequency range that a standard modem utilizes.
Of course, hacking it to connect to the mailstation is another story! =P