The phones could just use the SIP protocol for voice. You don't need an accompanying chat app. The huge disadvantage of that is that the customer could then use any VOIP supplier for their phone calls. ... or none at all for SIP to SIP calls. That would create a competitive market in voice. A competitive market is the last thing any cell phone provider wants. They need to maintain their monopoly on the last mile connection.

This might be an interesting idea for a government interested in improving this particular market. Forbid the provision of anything but reasonable latency data by a last mile provider. Establish standards.

I note that jmp.chat, a provider of SMS and voice over internet connections, is toying with the idea of providing, pay as you go, data only plans as a complement to their primary service.