No, voice & SMS are not always there. The GSM radio can be toggled off and use BT or WiFi exclusively. If you had read one of MY earlier posts you would know that 3g/4G/LTE will consume a lot of power if it has to hunt for a signal because of spotty coverage. GSM power consumption has everything to do with strength of signal so what works for you or me, may not apply to the next guy.
How do you turn the GSM radio off (without putting the phone into flight mode)? It's the thing that keeps you connected to the cellular network.