Hey HN — I built Stella Foster (stellafoster.com).
Stella lets you access your iMessages, WhatsApps, and Telegrams from any phone — flip phone, feature phone, anything. A Mac desktop app connects to your messaging channels natively. Once that's running, Stella has a phone number — you text or call her from any device and she gives you your messages.
No apps or internet needed on the second phone. If it can call or text, it works.
Free to try. Happy to get into the technical details in the comments.