Any thoughts on building your own phone?

Recently I had the idea of building my own phone with a Raspberry Pi. I started searching and I found this article where someone had already done this. Unfortunately this article is from 2014 and I can't find a newer article with a functional phone with SIM card. At some point this guy says he's running Raspbian. Are there mobile desktops for Debian? Mobian doesn't look very real.I don't want Android although Lineage/etc might be acceptable... (Although now I'm thinking of putting Droidian on an old phone...)

Anyway does anybody have any info, thoughts, or advice on this? Am I a total idiot for wanting to attempt this?