I don't know of any T-Mobile predictions that say you will have "excellent coverage, even indoors." In "excellent" coverage areas T-Mobile predicts "in most homes and big buildings." It's always possible to build a structure that will effectively block radio waves. If you have good service outside, there's really not much to complain about. If you are still paying on your iPhone you might be able to invoke the Lifetime Coverage Guarantee which would allow you to unlock your phone, port to a compatible carrier (e.g. AT&T) and continue making just the regular payments on your phone. If your phone is paid for, just unlock it and port out.
magenta8869103 Office buildings that are large can prove to be difficult when it comes to getting strong signal What does your coverage look like when you are outside of your office? Does your office have a WiFi connection you can use while you are there?