I bought a volla years ago for ~350€ iirc.
Before buying I asked their support e-mail whether it had DP alt mode via USB C. The CEO replied and assured me that they do.
Bought the phone, fucked around with it for months trying to make display output work (the return window passed). After more back and forth, find out that they actually don't have DP alt mode... ask for refund, they make me ship the phone to them, say that there is a scratch on the cheap plastic back and that they could only refund 100€ because of that... I declined and sold it on e-bay.
The phone itself felt very cheaply made as well. But maybe the newer ones are better, idk.
Take from this what you want.
A different linux phone I thought looked cool recently is: https://linuxiac.com/liberux-nexx-linux-phone/