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Look, it's hard to get a good quality VoIP call in Australia using a mixture of 3G & 4G LTE.

You're oversimplifying it. The low bandwidth requirement is a positive thing for everyone. Mobile towers and networks share bandwidth between all connected users at that point in time and the available bandwidth can fluctuate a lot. This can be based on people moving, penetration of signal, device radios, etc. The more bandwidth required the more you'll need to complete a round trip successfully "eg a transaction" - voice traffic uses a real time protocol and as these changes in bandwidth directly translate to dropped words. It is not comparable at all to web traffic which can re-send lost information along "the pipe". Needing very little space "on the pipe" to complete the transaction from a to b makes a remarkable difference to the user experience.

The trip from A to B in most cases is phone to phone and during that time your conversation might pass through 20 different devices and at ANY point data can be missed or dropped. ZipT is A - B (Server) does some fancy stuff to C. A-B is a shorter path and less can go wrong, B-C has better data sequencing and missed information can be resent. That's why you get a better experience, but with a bit more latency.


Edit: Clarification

A-B Is your typical peer to peer voice app where the voice is directly sent through the data network from device to device
A-B-C is where there is a server in the middle to handle the transaction (eg similar to ZipT)