Are you talking about using your phone to give your xBox an internet connection? Then yes, you can. Not sure how T-Mobile does it, but most carriers offer a tethering plan (which includes the mobile hot spot support found on many smartphones, especially Androids. This usually is NOT unlimited, unlike your on-phone data, and carries an extra fee (often capped at ~2GB). Alternatively, you could use a 3rd-party app, which usually requires a one-time purchase/activation fee that allows free (unlimited) tethering. However, be wary of this, because (a) it can potentially void your warranty because carriers don't like giving out free data and (b) if you are running your xbox on it, then you will likely use huge amounts of data, and your carrier will probably notice and potentially shut you down.
Another thing is, unless you have a 4G device, I generally have problems with my ping through my phone. Usually I end up with ping of 110-200ms, which is probably too high for fast-paced games like first person shooters.
Hope this helps.