Depends on trust level. Raw transactions or sending private keys is two options that come to my dumb mind at the moment.
SMS is plaintext and insecure by default, so sending private key is idiotic idea from beginning that just might work.
That could work. Since there's no proven encryption for SMS/Text messaging, sending a raw transaction through the phone would be the wisest thing to do. The only issue would be that the receiver needs to have an internet connection in order to broadcast the transaction successfully to the Bitcoin network. If there was a way to send BTC transactions offline using SMS/Text messaging, then it would've been a blast. After all, not all countries of the world have internet access. The Blockstream satellite helps, but the user still needs an internet connection in order to broadcast transactions over the Bitcoin blockchain.
Maybe someone decides to create a relay/gateway server that would connect to a landline and provide BTC wallet functionality to phone users? Users would interact via a bot over SMS/Text messaging (similar to how it's done via Telegram) with a set of pre-defined commands. The bot would then communicate to an SPV Electrum server to manage key pairs, create wallets, etc. At least, the idea is there. But making it a reality is another story.
Did you mean sat2.io ?
I haven't tested it by myself, so obviously use at your own risk.
Interesting. Never heard about this before. If the service works as intended, people will be able to send BTC transactions over SMS/Text messaging. The only downside is that the one sending the Bitcoin needs to have an Internet connection to access the service. Personally, I wouldn't mind using this service for P2P BTC transactions using a featurephone. But I would've loved a way to send BTC transactions offline using this method too. If made possible, Bitcoin's adoption within the mainstream world would grow like never before in third world countries. After all, not everyone has access to the Internet these days. But I guess that developers are focused on other important things for the improvement of the Bitcoin blockchain besides this.
you can't not need a third party in such a design. lets take a look at two situations:
1. sender doesn't have internet but receiver does
this is a much safer situation. all sender has to do is receive his transaction history from a third party using any other method like on a floppy disk! then he creates a new transaction sending bitcoin to the receiver, all offline and signs it. then sends the signed raw tx using SMS. then the receiver (having internet) broadcasts it to be mined, he can continue monitoring the situation with his tx and know when it confirms, if it is valid,...
worst case scenario in this case is if in first step the sender doesn't receive all his UTXOs or receives wrong ones. in any case he won't lose any money.
2. sender has internet but receiver doesn't
this case is very different from the last one. the sender has a much easier time since he has access to internet and can create the tx and broadcast and monitor it. but there is no need for him to send the raw tx through SMS to the receiver since he doesn't have internet and won't do him any good. all he needs to do is to send them the txid (hash) and maybe the block height.
now the receiver has a much harder time because he has to find a way to verify if the txid is real. what they can do in order not to rely heavily on third party is to receive block headers and the merkle root of the same block height that the txid is in. they can receive it through snail mail in a USB disk or burnt on a CD/DVD. since block headers can't be faked without spending billions of dollars worth of hashrate and merkle root can't be faked thanks to SHA256 they can be more sure about honestly of the transaction they received.
Very nice observation. Thanks for presenting different scenarios about how sending BTC over SMS/Text messaging works. Honestly, I'd wish there was a way to send/receive BTC transactions without middleman in a safe and secure way using any phone. This would allow people who live in areas where there's no Internet access to easily interact with the Bitcoin blockchain. But achieving this via an unsecure protocol, seems very unlikely. With other ways to send BTC transactions without middleman in an offline manner (satellites, mesh networks, radio waves, etc.) there might be no need for something like this. Still, the idea is there. It's up to someone to come up with a decentralized solution that would fulfil said purpose.