A really big reason are signatures and laws. For example in some countries you can just fax a signature while using digital signatures would require you to get an official digital signature (usually stored on a smart card, if it exists in the first place, and sometimes having less "validity" than a handwritten signature), then get a smart card reader, install middleware which may or may not work with the program you wish to use to sign the document, and then the receiver may also need to have program which will work with the digital signature to confirm it, and so on for the whole chain.
It's not uncommon for some businesses to need to submit signed documents to dozens of different government regulatory offices, agencies, banks and so on and in all of them everyone will have to be trained and equipped to use the digital signature. Such preparations may be made on a level of single country, but what happens if you get a signed e-mail from another country (when the signatures are even more important because the other person can't just come over and sign a document himself and mail takes time and money).
Also the whole process of sending authenticated mail is a bit complicated. For a simple office clerk, it's often easier to just use fax, and then focus on doing the job he's supposed to do.
So basically the main reason is inertia of the society. Everyone needs to get used to e-mails, and almost everyone is already used to using a fax, so transition isn't easy.
Another problem is how difficult it is to access documents. Using a fax, you can easily send handwritten notes, or take a document and underline it by hand and so on. On computer such "simple" things are a bit more complicated. For example, if you have a document on the computer, you'd need to print it, do editing by hand and then scan it, or edit it on the computer, and people often aren't as used to interfacing with computers as they used to interfacing with a pencil. Also, mice are difficult to use for drawing and graphical tablets aren't often a part of standard office computer.
From personal experience, I prefer Google Voice. The service will allow you to port over your existing phone number to Google. The features it has are awesome: voicemail transcribing, massive contact support, SMS, and so many more I can't list. The awesome part is that you can make Google Voice ring multiple phones. It could ring a cell and a home line, or even add a work line (you can select time ranges to ring certain phones).
This setup will cost you nothing per month, and it can be connected to a normal home telephone:
Google Voice and the Obihai OBi110
The OBi110 connects to your Google Voice account over the internet. Then it talks to Google's servers and it will translate that into a normal telephone line. Setup is easy and the device is inexpensive. You only need an internet connection, Google Voice, and the OBi110. The OBi110 is also on sale at Amazon.com right now for $49.99.
Here's how to set-up the device once you have bought it.
Best Answer
You can use a Voice over IP analog adapter, but there are two important caveats:
If you don't have or can't setup a VoIP PBX, you can buy appliances that make it easy... but even simple ones usually run a few hundred dollars. You might find something like the Nettalk duo more to your liking.