M-Pesa has demonstrated that payments via SMS are a viable solution for those that lack internet access.

GeoPay and others are attempting the same but utilising blockchain.

http://bitcoinafrica.io/2017/08/02/africas-blockchain-startu...

https://btcmanager.com/samourais-new-feature-send-bitcoin-vi...

The problem is not just not having access to banks. Try to send money across the world living in the third-world. There's stuff like Western Union in may places, but depending on where you are there are extreme levels of red tape and very low limits on how much and how often you can do it. Also whenever there is unusual volatility in the market, that kind of service will stop servicing. And good luck setting up a US (or EU) bank account remotely from 80% of the world that's not developed.

How difficult would it be to get cell phones to the people who currently don't have them?

Cell phones are exploding in the developing world.

Probably quicker than setting up banks?