They actually did that with the SimplePhone. Last time I looked the pages were down but I know I archived them on the wayback machine.
They made lots of promises, including that a downloadable ROM would be available, the whole thing quickly crashed and burned.
Reddit had some pretty extensive threads about it. The whole thing seemed like a grift (it was) and convinced me to dump all their apps.
Anyway, if this is something you want to do you could check those threads, I know people linked to the phone they were using (from Alibaba) and the source code was on Codeberg iirc.
There is Open Contacts which also acts as a dialer if you want it too. It creates its own database which could be useful if you don’t want other apps to have your contacts.
It works fine, I use that or Koler