Presumably the reason for SMS is because the recipient is not a Signal user nor a user of any other encrypted service which is trusted by the sender. In that case one would want to still use encryption provided by OpenPGP or other trusted alternatives. If the recipient is an OpenPGP user then this is a good enough solution. An alternative, if both parties can be persuaded to adopt it, is https://silence.im/
thanks for clarification. but … I still have a problem to ‘understand’ If I use SMS encrypted via PGP because the receiver doesn’t use signal or any other encrypted messenger, isn’t it easier that the receiver will use also an encrypted messenger ? I don’t know how much encrypted messangers we have, but I think here will be one which can be used from both.
Yes. However, it is necessary that both parties trust the means of communication. In this instance I suspect that OpenPGP is the only part of the chain that both parties trust.
Furthermore, SMS means disclosing one’s phone number only to the phone network provider (unavoidable.) Whereas OTT messaging services usually also require the number. In this case the recipient might not be comfortable with that.