I have made lots more calls on ZipT and measured the data usages...

I have made lots more calls on ZipT and measured the data usages on each. ZipT's data usage is very consistent- the range is between 160kb/min to 180kb/min on all counts. Whereas Viber's data usage swings wildly to anything from 240kb/min to over 1Mb/min with most ranges around 700kb/min. Viber's voice quality can be very good and I think this is when they chew up data the most. Video call is pretty useless when I call Cambodia as they can hear and see me but I can see but not hear them. Frustratingly I can't call Cambodia using ZipT. Viber and Tango's problem seems to be when I ring a number that is inactive I can still hear it ringing as if the call made it through.
The problem I find with ZipT's calls is that the voice delay is longer than that of Tango and Viber. I don't know much about VoIP technology so I wonder if eventually the scaling up of servers around the world will improve that or whether this is the compromise we have to put up with by utilising lower bandwidth.
Any techies able to clarify that?