I don't believe that you can send and receive faxes from your cellular phone using the traditional methods of sending and receiving faxes.
However, there are methods to send and receive faxes over the Internet, and one company even offers a cable so you can hook up your fax machine to your phone so that it can receive the fax via Internet (so you can print it). This seems a bit unnecessary, however, given that you could just as easily open this document on your computer and print it there using your printer. But this fax to smartphone hookup is only good if you use an Internet fax service to send and receive faxes online.
Here is one of the many companies. This one seems to be the most reasonably priced for the paid sites.
But there are various other options for doing this. Some are free. Some are not. Most of the free services are very limited and won't actually allow you to receive faxes. There are several that claim to be free, but if you check, very few actually are.
To send a fax, you would just need to create a file. If you have a piece of paper that you need to scan, there are apps on the Android Market that will allow you to "scan" documents using your phone's camera. Just make sure that the app has the ability to "scan" multiple page documents so you can send all of the pages in one fax job.
This method (of Internet faxing) would also allow you to be mobile with your mobile device, being able to send and receive faxes without the need of a fax machine. Only a printer for printing the documents that you would receive (if it's a form you need to fill out and return).
btw, there are several apps on the Android Market that allow you to fax via the Internet, but they require some sort of payment for services (even though sometimes the app is free).