Looking at postmarketOS devices page only 20 community supported devices have "Calls" listed as "fully implemented". Clearly this is the main hurdle for true "Linux Phone". And those devices are a bit old, ranging from 2014 to 2020, and that makes them harder to obtain, also battery might be in need of replacing. And Pinephone is currently out of stock. Cannot comment about Librem 5, but its price is quite high. So, ignoring my silly idea of "PDA" - which Linux phone should I buy and how could I contribute to its success?
> only 20 community supported devices
You realize it's a result of reverse-engineering done by volunteers? To me it looks like a large number actually. The main hurdle for true "Linux Phone" is the lack of free drivers and specs, such that volunteers must spend tremendous efforts to make basic things work.
> which Linux phone should I buy and how could I contribute to its success?
To help Linux ecosystem on mobile, you could support vendors who intentionally provide free drivers or specs and help improve the software running on their devices. AFAIK only Pinephone and Librem 5 fit here. The former is in stock from time to time.
Also many users of GNU/Linux phones run Phosh developed by Purism. Helping to improve it would help the whole community.