The AT&T model of the MPx200 appears to have been restricted to only two bands – 1800 and 1900.. No 900Mhz. However, in this MPx200.org story, there IS a solution which allows you to download a different version of the Operating System. It also appears to add in MMS messaging and camera drivers too.
As always, we would suggest caution if altering your Operating System version, as updates will usually check your software version….. i.e. you could be screwed if you want to update to another version later.
Source – MPx200.org