Naven
Answers
Does Naven work on an old phone?
If the browser is reasonably current, yes. There is nothing to install, so storage is not the constraint it is for an app — the limit is whether the browser supports the features calls need.
This is a genuine advantage of a web app on older hardware. An app store often refuses to install recent versions on an old operating system; a web page has no such gate.
Voice and video calls need a browser with WebRTC, which has been standard for over a decade. A browser more than a few years out of date may manage messaging but not calls.
A handed-down phone with no SIM works fully, since no phone number is involved.