WhatsApp connects calls over the internet between two devices that both have the app installed. You open a chat, tap the phone icon, and the call goes through your data connection or Wi-Fi. This works well when both people already use WhatsApp daily.
Talkala works differently because it connects your browser call to the actual telephone network in the destination country. You type in a phone number, check the per-minute rate before you dial, and call the person directly. They answer on their regular phone like any normal incoming call.