If you have unocked your MXP200 and wish to use the T-Mobile service, as I have, then you can use the following settings. If you need further details on how to do this, please let me know.

T-Mobile USA

GPRS CONNECTIONS:
Description: T-Zones GPRS
Connects to: The Internet
Access Point: internet2.voicestream.com (or internet3.voicestream.com)
User Name:
Password:
Primary DNS: 216.155.165.50
Secondary DNS: 216.155.165.50
IP Address:

Description: T-Zones WAP
Connects to: The Internet (or try directly to WAP network)
Access Point: wap.voicestream.com
User Name:
Password:
Primary DNS: 0.0.0.0
Secondary DNS: 0.0.0.0
IP Address:

PROXY CONNECTIONS:
Description:T-Zones WAP Proxy
Connects From: The Internet
Connects To: WAP Network
Proxy (name:port): 216.155.165.50
Type: WAP
User name:
Password:

Settings-Data Connections:
Internet Connection: T-Zones GPRS
WAP Connection: T-Zones WAP

INTERNET EXPLORER > MENU > OPTIONS > CONNECTIONS:
Tick 'automatically Detect Settings'

MMS setup
Settings-Data Connections - Edit GPRS connections
Description : T-Zones MMS
Connects to : MMS connection
Access point: wap.voicestream.com

Settings - Accessories - MMS setup
First field: T-Zones MMS
IP: 216.155.165.050
Port: 9201
Mmsc Address: : http://216.155.174.84/servlets/mms