It's possible you need to set your APN settings.
Go to options > advanced options > TCP
There you should enter just as shown below:
*actually leave blank
Note: T-Mobile has fixed the port-blocking issue. In rare cases, it is reported you may need to enter your T-Mobile username and password, the same information used for logging into Unsupported Browser
... If it still does not work properly, just call T-Mobile (call after Feburary 11th, 2005), and ask them to get the "wap.voicestream.com Internet Access Point Name" added to your BlackBerry account at no extra charge.