said by RARPSL
The UMTS/HSPA+ is their 3G/4g DATA (ie: Internet) Network. The Phone and Text are 1900 Mhz (which is why the iPhone can work as a PHONE with T-Mobile SIMs/Mini-SIMs).
UMTS/HSPA+ can actually handle voice and SMS. For example, I turned off my 2G radio in my Nexus One and am only using AWS 1700/2100 for everything.
In contrast to LTE and WiMax which at this time cannot handle voice. (In any case, they're likely to use a VoIP/SIP based service in the future).