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This correlation is not always true, an in emerging markets, this correlation is almost always wrong. Access to Smart phones unfortunately doesn't always mean the availability of banking infrastructure OR the presence of a cashless economy . For example -

- Android phones are dominant in this market, since local manufacturers are able to sell cheap phones without incurring any software development cost.

- Telecommunication sector is heavily privatized in these markets & generally have multiple players offering low cost "pre-paid" services. Recharge stations and mobile stores are pretty ubiquitous - small mom & pop shops double up as resellers of these services , providing last mile coverage.

- On the contrary, Banking infrastructure is so heavily state run. And they don't have the same level of 'ease of access'. So, a lot of people working in the "unorganized sector"(Note: This is an official terminology that governments recognize and use) don't have formal accounts in financial institutions.

- Even if they did, a good number of banks run decade old tools and softwares. Making it all the more difficult to do mobile & online banking .

- And also, most of these economies are not cashless. Culturally and for other reasons,there is more trust on "Paper Cash".

- Finally , just because a person owns a cheap smart phone & ,one cannot assume banking literacy.

In my country most cell phones are on pre-paid plan. You can buy cell phone credits at a gas station, then that credit gets used up as you make calls and send messages.

When the cell phone companies came up with a service to transfer cell phone credits to other users, people started using that as currency. Only problem is each company runs its own proprietary system and there is no way to send credit to a someone who uses a different cell phone company.

If you think about it, phone companies have the best micro-transaction infrastructure on the planet. They are constantly charging people fractions of a penny per second per phone call.

I use cash as often as possible at small and local businesses, because then they don't have to pay the 3-5% fee to the debit/credit card processing company.

That's true, but non-volatility counts for something there. Money classically has three roles, as a medium of exchange, a measure of value, and a store of value; when something's value fluctuates dramatically over time, it's failing to fulfill the second and third roles. Measure of value is unimportant for day-to-day use (although vital for accounting), but store of value is indispensable.

I'm guessing that we're secretly talking about Bitcoin here; I think that what BTC needs is some organization that controls a lot of capital, and is willing to underwrite the value of the bitcoin. Fiat currencies work to the extent that their issuing countries can underwrite their value (by accepting the currency in payment of debts, primarily); precious metals work to the extent that there's non-monetary demand for them (so gold isn't actually very valuable in an apocalypse -- look at prices in the Siege of Sarajevo, discussed in a survivalist blog a few years back). Bitcoin doesn't really have either of these. It's a sort of fiscal hot potato -- good for exchange, certainly, but you wouldn't want to be the last person with all the world's bitcoin if everyone else lost interest in it.

Web of trust doesn't have a centralized determinator. You assign trust to people you've interacted with, and they assign trust to people they have interacted with, etc.

Putting it on a blockchain doesn't make that more effective.

If you look at the reviews of strangers, you have to figure out which ones are real and which ones are fake... it works exactly the same with or without blockchain.