What is the hardest interview question T-Mobile asks?

None of them are really hard. They are based on the SAR method, Situation, Action, Result. Typically you are asked to describe a particular situation, what action did you need to take and what was the end result. One example would be "please describe to me a time in which you had to work with a difficult customer. What was the situation and why was it so difficult?" A possible answer would be a customer came in because their phone was not working and it was apparent there was physical damage to the phone. The customer refused to accept that they would have to buy a new phone so I provided them the option of filing an insurance claim, which they did not even realize they had. I assisted the customer with filing the insurance claim and they left very satisfied because their replacement phone would be arriving the next day.