Dumbphones are not inherently more secure because they’re “low tech”
Dumbphones are not inherently “safer” than smart phones for being “lower” tech. The opposite, in fact. SMS is not private at all. Everyone from the carrier to feds get to take a peak at the data/metadata and even silently censor messages. Same for phone call metadata.
If your motivations for a dumbphone are privacy, you’re better off with a smart phone that let’s you use privacy-focused, audited, open-source software like GrapheneOS, Matrix, Signal, etc. The software and protocols on regular dumbphones are old, outdated, and the security & privacy benefit is dubious at best.