PinePhone, running KDE Plasma Mobile Linux UI, for $149, might be out by 4th Quarter 2019.
https://liliputing.com/2019/04/first-look-at-the-pinephone-dev-kit-running-kde-plasma-mobile-postmarketos.html

Looks promising, that company has done a lot with low cost SBCs.  I think it is important for them to keep the cost under $150; if it is too high then there wont be enough adoption.  Personally, I'd love to have a true Linux powered phone.

There are several Linux OS's for phones, but having to install them yourself on an old phone just isn't going to bring about wider spread adoption.  It's too scary for most people.

Having a phone for sale with a Linux OS is a great first step, like a proof of concept.

I have a Pur.sm Librem 5 Linux Phone on pre-order.  They keep moving the release date back but currently it's set for 3rd quarter of this year.  It's pricey but I want a little more horsepower than the PinePhone.

With desktop use falling away, Linux needs to get into mobile to continue forward, and we need Linux to breakout of the Android and iOS monopolies on mobile operating systems.

A little more info on this:  it looks like there will be several "builds" each with a different Linux based system.  Most notable are:

1. UBPorts - the continuation of Ubuntu Phone project.
2. Sailfish - formerly Nokia's Linux based OS abandoned when they got Borged by Microsoft.
3. KDE Plasma - previously listed
4.  Several others.

https://wiki.pine64.org/index.php/Project_Don't_be_evil

Love the Project Name "Don't be Evil".  I wonder what they are referring to?

The Linux-based PinePhone is the most interesting smartphone I've tried in years