Came here to say this as well. Actively considering selling my Samsung S20 for a pure linux phone. Something like https://store.planetcom.co.uk/products/cosmo-communicator
My understanding is that the Pinephone, Pinephone Pro, or Librem 5 are the only pure Linux phones available... and the Librem 5 is 'barely' available given the long lead time on orders.
Technically, Maemo Leste is a pure Linux option on some older handsets, but I'm not sure any of them support VoLTE, so increasingly that would mean no telephony support... depending on where one is in the world and whether they've dropped 3G and 4G non-LTE calling.
I would love to know if there's other pure Linux options.