@bernard_parah referenced USSD during his AMA as an ideal way to connect to Lightning from countries dominated by feature phones. I got curious about it ... You should be able to build a custodial wallet on USSD at least AFAICT.
I just happened to see this video:
Get and transfer Bitcoin via USSD without internet - by Atsu Davoh (A test demo. Not the real app).
It's using Testnet bitcoin, and the USSD gateway is a "Shared USSD" which is why it has the second code (the "*88" part of the "44788#" code that was dialed). A larger service in production would want a Dedicated USSD gateway, so users enter just the three digit code "*447#".
Also there are costs for each USSD message (ranging from ~$0.01 to ~$0.03, and the party paying may be the service provider or the end-user, depending on if the end-user is on a pre-paid or post-paid mobile network subscription).
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I learned of that from this thread:
Might mess around and build that thing that creates a stablecoin address for every mobile money number.
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A Tweet in that thread shared this, which I also had not known of before:
Oblivious Decentralized Identifier Service (ODIS) https://docs.celo.org/celo-codebase/protocol/odis
Celo is a L1 blockchain project, with CELO token, and then the following stablecoins: cUSD, cEUR, and cREAL (BRL)
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Requires provisioning from the mobile network telecom provider. In Nigeria, they are adding USSD support for the e-Naira (CBDC), so that it can be used from feature phones.
The problem is, in many developing countries, the government either owns the mobile network companies (in full, or substantially), or controls them via either regulation or edict. And if the government in a country is hostile to bitcoin, they certainly won't be permitting provisioning of USSD for a bitcoin-related endeavor.
There's another approach, known as Sim Toolkit (STK). Same issue, ... the SSL certificate for the service must come from the network telecom provider, which they likely won't do for bitcoin-related endeavors.
So that's the choke point.
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Coincidentally,
Machankura - LN custodial wallet accessible via any phone in six African countries #40804 https://8333.mobi
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