Comment Re:"It's the Network" (Score 2, Informative) 745
The bands for 2G (GSM/EDGE) and 3G (W-CDMA) are different, so you can have a phone that is "quad-band" for 2G but is single-band for 3G. The radio designs for W-CDMA are MUCH more complex than GSM/EDGE, thus having a phone that supports multiple W-CDMA bands significantly increases the BOM cost and the radio footprint compared with a single W-CDMA band design. Such phones are definitely being made, but at least it makes sense why a phone targeted for T-Mobile would only support their frequency bands.
FYI, in the States, AT&T uses bands II and V while T-Mobile uses band IV. See http://en.wikipedia.org/wiki/UMTS_frequency_bands