I do not know what operating system the feature phones of these impoverished nations with little internet are using. My best guess is Java Mobile Edition (please confirm is you happen to know)
Why make an app?
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SMS is not secure. SMS messages, like email, is much like sending a letter via the post office. You need the "To:" address to be correct so that the message reaches its destination, but if you don't need a response, the "From:" address can be anything. I very well could send you a text message to you and have you believe that it came from your own phone number, you just wouldn't be able to reply.
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Centralization. SMS to a central server is very much a custodial solution. The user must trust that the central server is never hacked, rug pulled, bankrupt, or shut down by government forces. By allowing the user to sign the transaction on their local device, the user is self custodying even if they must trust someone else to transmit that message to the rest of the network for them. A signed transaction can not be modified for a higher amount, or to go to another address.
Proposal
Create a feature phone application which stores Bitcoin private keys, and transmits signed transaction messages via SMS.