When writing a new message in Outlook, there is option to send message to
fax instead of e-mail address, if recipient has fax number specified.

"To..." dialog looks like this:
Some person 1 (E-mail)
Some person 1 (Business Fax)
Some person 2 (Email)

Is it possible to have other devices in this dialog, like mobile phones?

We are planning to write our custom event handling on Exchange 2K. When message
was sent to mobile phone, we will forward it to recipient using our own SMS
sending engine.