I think making an “usable” phone (especially one that is able to make calls, etc.) yourself is extremely hard to do, if not impossible.
Many “tinker”-phone startups/ devices, like PinePhone or Librem, who made the phones from scratch or mostly themselves afaik, had huge problems in the beginning with basic functions, like making calls.
There’s a project (mainly for kids and students) somewhere to make E-readers themselves, maybe you can start with that?
I’ll link it to you if I found it.
That might act as a base.
If you want a good phone that gives off DIY-vibes (modularity, repairability, etc.) but want something proper and modern, then check out Fairphone. Afaik, the FP4 also supports PostmarketOS and other mobile distros.
Remember to take everything I said with a huge grin of salt, since I’m not that well informed in that area of DIY- or Linux phones. A lot of what I said might be wrong, take it only as idea or starting point.
But if you really want to start this project, good luck. You’ll need it 🫠