normal calls go over the network as a call.
Everything that involves streaming counts as data. If its a video call or streaming a radio station off the internet (via GPRS/3g) its data.
Hence Datacall :>>
Also note that if someone calls you via a Video call. You and him both pay for that Video call, but if u pick up, and your phone feeds data to him via Video call. You also pay for the feed going in and out of your phone.
example . He/She calls you, and talks for 3min. he/she wil then be billed for a 3min video call, but you will also have to pay exactly the same rates for that same 3min call. So mtn/vodacom/cell-c will infact get payd twice for a 3min call.
Somehow they don't always tell u this kinda stuff