How Can I Find Out the Number of Telephone (Pstn) Users World-Wide.?

Well, it's really hard to be exact, but wireless phones hit the 2.5 billion mark in August 2006:
http://www.cellular-news.com/story/19223.php

http://www.eadp.org/index.php?q=node/14752

This article gives figures on VOIP numbers:
http://www.technewsworld.com/story/54807.html

There are at least 2 billion landlines, according to this article.
Although it shows it was posted in Jan. 2006, it talks about voip projected growth for 2005, so it has to be older than that.

http://gigaom.com/2006/01/31/voip-not-just-for-cheap-calls/

Landlines are decreasing every day, so the number is still proabably pretty close. Landline useage for businesses is expected to start picking up again.

Hope this helps a bit!

Polly