Technically you are correct. Flexpay Contract customers are limited to 2 lines of service. Flexpay Month-to-Month customers are limited to 3 lines of service. There are ways for activations to get around the 2 line limit to allow a contract customer to have 3 lines (I have two customers that have had that happen but both were Class-A credit lines).
Congrats to the OP on getting the problem solved. I have noticed more often that when I call in to dealer or customer care that the first level person I speak to doesn't seem to be in the US but if I get transferred to a level 2 agent they generally seem to be (some are right around the corner from me at their home office in DFW). T-Mobile seems to have been slowly and quietly following the overseas customer care approach everyone else is currently using. I hope they can maintain their normal levels of care since that is the biggest thing that has them standing outside of the crowd.
Zo