I tried plugging my XT SIM into my old SE W900 which only supports 2100Mhz, and while it sees the TelecomXT network when I'm in range of the 2100 tower(s), if I try actually connecting to it I get a "forbidden network" message. The phone is completely unlocked.
I was only trying it out of interest to see if it would actually connect, should it work? I know the phone won't be any actual use due to not supporting anything other than 2100Mhz, but it was just an experiment.