GNU/Linux phones exist: Librem 5 and Pinephone. They're based on free drivers and don't prevent you from installing any OS.
This is on the Pinephone Pro btw.
That you then can't really do anything with either because of the super limited HW support (due to only using "free" drivers) or because they're so slow you just give up in frustration. Or lack of apps.
At least in the case of the Librem 5 its wildly overpriced for what you actually get (if you ever actually get it that is).
If you think the hardware is slow, have a look at SXMo.
> if you ever actually get it that is
They've been delivering the phones within 10 days since a long tome already. Sent from my Librem 5.