Making and receiving phone calls do NOT use mobile data. You can disable mobile data and you can still receive/make phone calls (if you got good cellular connection). If you don't have good cellular connection, you can still make/receive phone calls thru WiFi if you got 'designed for Fi' phone.

Do phone calls use cellular data?

Phone calls do not count as "data" but do, obviously, count against the minutes on your plan. It is important to note that while connected to Wi-Fi, only MMS uses data. iMessage, Apps, mail, web browsing, etc, all utilize Wi-Fi over EDGE/3G/4G, while MMS does not, regardless of Wi-Fi connectivity.

How much data does a phone call use?

Per minute (video call) = ~3.25 MB. Per hour (video call) = ~ 195 MB. Per minute (audio call) = ~2 MB. Per minute (audio call) = ~ 120 MB.