It is worth it to note that SMS should not be used for any personal communication. Literally every message you send is stored by the carrier and handed over to anyone that pays them including LEOs, who in the past have used them to incriminate innocent people, and if you live in america, they will use them to justify shooting you apparently...
Not to mention anyone in the area can intercept all messages with some cheap equipment.
Dumb phones are such a draw compared to stupid smart phones, but they don't have any secure messaging (at least the ones I have looked at)