Searched on Dell.com and found
Latitude D620 Product Details:
Wireless:
Dell Wireless 5500 Mobile Broadband 3G HSDPA (Cingular US)
Dell Wireless 5700 Mobile Broadband CDMA EVDO (Verizon Wireless US)
So maybe it didn't work because you have T-mobile and not Cingular or Verizon.
Other google hits showed primarily UK sites:
Trusted Reviews
Reghardware.co.uk