I can imagine that commercial interests may try to stop the concept of free phone calling using voip by denying the use of their cell phone infrastructure or by raising prices on internet-only service.
It seems that with Purism doing all of the heavy lifting here, that it might be relatively easy to put an alternative radio/modem in the Phone and use alternative frequencies and a ham radio license by the users (to keep it legal), to bypass any limiting commercial interests, legally. Someone would just need to write an alternative application that accesses the radio/modem.
There is an exponentially high number of available frequencies in the GHz range that are available for amateur radio use (compared to 2m/440). Anyone with an amateur radio license can legally throw up an antenna and a radio/VIP bridge to the internet. Now, a phone that gets only wifi-voip but also can do amateur-radio (auto-patch) calls with a virtually unlimited amount of available frequencies for outgoing calls might be a tempting prospect, especially if you never get a phone bill. There are probably a thousand amateur radio / code writers who would go to work on this once the hardware is available to them.