At this stage, I believe you now have to telephone T-Mobile to make sure that the wap.voicestream.com is properly added to your account. Sometimes you have to do this.
Make sure you contact a BlackBerry tech support representative and ask for this quote "I am calling about my phone. I need to have something added to my account. I need the wap dot voicestream dot com Internet Access Point Name added to my account." (or say something similiar).