If you need a tutorial on serial modem communications, a good place to
start would be:
http://www.arcelect.com/rs232.htm
That page is heavy on the general stuff and the cable pinouts, usually
what you need first. Look at the links on the bottom of the page for
more pages having to do with serial communications, and phone modems in
particular.
As always, the Jedi master for serial communications tutorials for
newbies is Jan Axelson of Lakeview Research, and the Book "Serial Port
Complete" by that author. You can get it from libraries, Amazon, or
the Lakeview Research website
http://www.lvr.com/serport.htm
This webpage has links to everything. But if you really want to learn,
try getting a hold of the book. It's great.
If you could be more specific about your needs, you might get a better
answer. I'm not really sure what you want.