The phone will have normal GSM in it, as well as being Matrix native when on IP (i.e. shipping a native Matrix.org client as the default dialler/messaging app, which will work whenever you have IP connectivity).
A stretch goal is to also provide Matrix<->PSTN connectivity, so that if you're on IP but don't have GSM (or don't have a SIM in the phone), then you could also place/receive calls with the PSTN from Matrix. (Obviously this would then be also available to the rest of the Matrix ecosystem too). (Source: I work on Matrix)