Motley Fool mentions ZIP, page-35

Whether you're comparing kilo bits or kilo bytes or mega tits or giant sausages for that matter, the fact is I have used the app and mearured the data used in a 1 minute time frame and compared it to Tango and Viber, and on average ZipT uses only a third of data compared to the other 2. Viber's data usage fluctuate wildly anything from 240kB/min to over 1MB/min but are usually around 700kB/minute. So get the app and test it for yourself, don't just pull figures out of the net.

It seems every VoiP company claims they have 'crystal clear' voice on their app, so I will leave that bit for actual life usages. But I can say that under good conditions Viber seems to have the best voice quality compared to others, even ZipT.

There are many VoiP apps (not countless) out there, but marketing it will be the key to success.