Even though your voice is magically being transported through the air, cellular phone calls actually work more or less the same as their land-line counterparts. T-Mobile is finally ditching the old-school circuit-switched method for Voice over LTE (VoLTE), which sends your sweet lyrical words via IP (Internet Protocol), just like Skype. The carrier will still be able to deliver its HD Voice feature, but it'll do so over the same airwaves that it's also pushing high speed data. That should eliminate any issues with using data while on a phone call and will actually mean your calls will connect faster. And calls should transition seamlessly if you lose LTE coverage, but still have access to T-Mobile's HSPA.