I haven't had a "land line" in multiple-decades. My short list of "the next thing" requires:
- Ability to install on a mobile device without any connection to a "phone number." Phone numbers in my mind are dead. - Skip the synchronous social convention of calling. Everything should be async by default. You don't ever cause someone's device to interrupt them. This is already culturally "accepted" with SMS. Unopened messages are deleted after some number of days. - With permission, these asynchronous messages should be forwardable. You give permission in advance of course, and then you can avoid having to repeat yourself again and again. - TMDA style whitelisting for unknown public keys wishing for contact. Addresses need some kind of seniority (older than 30-days) and some reasonable integration into the network at large (vouching connections between other people with "skin in the game", akin to a shared reputation--If I could remember the name of that early social network that tried to do this, avogato.org? Something like that.) - Mesh networking forever. Decentralize it all.
This is not something that I think can be a "startup," because this needs to be a protocol, not tied to any firm. If anyone's software works with the protocol, then let the "best installable" win. The closest thing to this right now is Scuttlebutt protocol and if someone could re-introduce contact sharing via "bumping" (using the accelerometer and GPS location and time together) devices together to exchange public keys, that particular bit of UX could go a long way to making it easier to adopt. However, the worst part of Scuttlebutt's capabilities has been the storage consumption for very popular identities that often consumes all the available memory on smaller devices. It needs data-storage hygenics--some way for the end-user to state what kind of data they want to keep and what they wish to ignore.