The version of the phone that visited the FCC had HSPA+ and LTE connectivity through AT&T. This particular variant did not offer HSPA+ frequencies that work on T-Mobile. There is a good chance that the HTC One mini won't be available through all four major U.S. carriers, so AT&T customers with an eye on the bite-sized version of HTC's Android flagship seem to have a leg up on those using other carriers.
The HTC One mini, besides having a smaller 4.3 inch screen, is expected to have a lower 720 x 1280 resolution, and a dual-core 1.4GHz Qualcomm Snapdragon 400 under the hood. There is some speculation that the phone is ready to launch in the U.K. on August 9th.
source: FCC via Engadget