First I would like to tell you a little about ‘how video calling works?’. (Jump to the end if you already do.)
*Since you’re asking this question I think it is safe to assume that you’re using cellular data on video calls.
Let’s say that you are on a video call with your friend. Now your phone’s camera records your video and sends it to a server. Then this video is downloaded on the other end by your friend’s phone, and is run(displayed) on his screen. Then simultaneously, the similar thing is done by his phone. His phone records his video and sends it to you. ALL THIS IS HAPPENING IN REAL TIME. Or almost real time. that’s why there’s very little or (sometimes) no time lag between sending and receiving.
Now the important part. How do these apps optimize your call in order to ensure that less data is used and also, that your call goes on smoothly.
They reduce the video quality. That’s why the video sometimes (or mostly, depending on your network) appears jagged or heavily pixelated.
They reduce fastness of video, i.e, Frames Per Second. That’s why the your friend on the other end (or mostly, depending on your network) appears to be stuck in one place; in such situations the FPS becomes really low, like 1–2.
They reduce the voice quality. That’s why the voice of your friend sometimes (or mostly, depending on your network) sounds distorted or broken.
So, consumption of less data compromises on the quality of the call. Some apps like Skype and Google Hangouts refuse to make this compromise, i.e. they only compress the calls to some extent, while others are more lenient. This means that Skype and Hangouts are a no, if you want to save data.
Apps such as IMO, Tango, ooVoo and some others are almost equivalent.
Here’s your answer: There does not exist any app which consumes ‘Very’ less data. But that doesn’t mean that you cannot get the most savings from any app that you’re using. This is what you can do:
Before doing video calls, restrict you system to use only 3G or even 2G, if the coverage is good. This would force the app(any app) to reduce video and voice quality and hence, reducing data consumption.
You can also try using OperaMax, a free data consumption and management app. Although I haven’t personally tested it with video calls, what I know for sure is that it wouldn’t have any effect on apps which use data encryption. So, if you can get your hands on an app which works with OperaMax, it’s your luck.
If you have an iPhone or an iPad, USE FACETIME. It is very good with optimization and surprisingly, uses really less data, considering the video quality. Heck! I have even used it with 2G. BUT CAUTION: Use Facetime only after restricting your network to 3G or 2G, or it’ll default to the best quality and you’ll burn through your data plan in no time. Only drawback is that the other person needs to have an iOS device too. How Much Data Does a FaceTime Call Use?