
Competitors
At the time of the release of the Ericsson T39 mobile phones developed rapidly. They were already small enough,
there is no need for a smaller phone. Instead of making the units smaller the number of features increased and
manufacturers seem to have put more effort in design than in the 90s and 80s.
Nokia had become a strong competitor to Ericsson and release the 3330, pictured to the right, which was an improved version of the best seller 3310. The Nokias appealed to many younger customers with it's large display, greater focus on entertainment and the 'pimp my phone' possibilities with a vast supply of exchangeable faceplates. And also, not to forget, it was a good phone.
Now, the Nokia 3330 was not a "business phone" like the T39m, nor the flagship in the Nokia range. However, not even the premium phones from Nokia and the other competitors were so full of useful communication tools as the Ericsson T39. Some were equipped with GPRS and WAP browsers, some with an IR-port, and some with both. But as far as I know, by the time of the release of the Ericsson T39m, only two phones featured GPRS, WAP and a true e-mail client. Those were the Ericsson R520 and the Ericsson T39, which made them the communication devices superior to all the others. And don't forget the world's first built-in bluetooth (well, second to R520), and there is absolutely no question about it.
Motorola was strong competitor in some areas, and Siemens mobile phones grew stronger. Also, Ericsson and many other IT and telecom companies suffered from the financial crisis the industry went through during the early part of the 21st century, and was soon (1 Oct, 2001) to be transferred into a new company, now well known as Sony Ericsson. The picture below shows different manufacturers yearly unit sales around 2000.

Picture source:
http://java.sun.com/developer/technicalArticles/javame/mobilemarket/