IMS, Not VoLTE, is the Future of Voice with T-Mobile

does that mean without the IMS-based client, you can't make phone calls over LTE on T-Mobile? Currently the wifi-calling client isn't a downloadable app because it requires a custom ROM and is only available on stock T-Mobile ROMS. Assuming that will continue to be the case, does this mean if I buy a future LTE 'nexus' phone or flash a different ROM on my LTE T-Mobile phone that I'm SOL if I wanna make calls over LTE?