Ok so there are good news for Volla 22:
@Bolly from the link you sent me I paste below the message from Jami Kettunen:
"the device page is just about finalized and should be merged soon-ish. and yes the UBports installer does have support to (re)flash Ubuntu Touch on the Volla Phone 22 with support sometime soon also coming to flash SailfishOS or Volla OS as well :)"

And here I paste the answer from Samuel (Volla support) in case it might be useful to somebody else.
"Ubuntu Touch is NOT the same as Ubuntu Desktop. The Partition size does differ from device to device (depending on the vendor images that will be used) You will be able to use a 256GB SD card for your secondary OS without any issues. Even 64 would be enough.The speed of your secondary OS depends on the speed of your SD card. However, I can barely see any difference between a system that runs on a SD card compared to Volla OS on the EMMC, even when it comes to launching heavy apps on Ubuntu Touch.You will be able to install Volla OS and Ubuntu Touch via the UBports installer."

@kugiigi Thank you for the answer! I did it following your instructions and it came back!
Yeah Volla 22 has the multiboot option where Volla OS runs in the internal storage and any other OS on the SD.
I'm anyway considering installing only Ubuntu Touch as I'm not a fan of android.
The only thing is that Volla 22 isn't listed among the supported devices but I know you can buy it with Ubuntu Touch installed as only OS plus from Ubuntu desktop there's the option to intall in on Volla 22 from the UB Ports application so I should probably go for it.
I'm very happy to finally have ubuntu on the phone! I've been waiting years for this moment! When Canonical abandoned the project it was a terrible news.

@muskist thank you anyway. Hopefully it'll be working when 20.04 will be relesed.

@keneda I did exactly this procedure, then I choose bluetooth, then I've got a screen with the picture I want to send, at the top left "x" to go back and that's it. Basically I can't go forward from there. Plus the device running UT is seen and used from the one running android as speaker.

@muskist yes I've read it too but wanted to give it a try. Is there any other way to transfer files via bluetooth?

Hi, I wanted to transfer some file via bluetooth to another device running android but it doesn't work.
I installed "Bluetooth File Transfer" which added 2 related apps ("Bluetooth" and "Bluetooth File Transfer") from the open store but I am unable to send and receive anything.
The other device recognize mine only as speaker (it literally uses it as a speaker) and there isn't any other available option on both devices.
Anyone knows how to sort it out?

Ok so there are good news for Volla 22:
@Bolly from the link you sent me I paste below the message from Jami Kettunen:
"the device page is just about finalized and should be merged soon-ish. and yes the UBports installer does have support to (re)flash Ubuntu Touch on the Volla Phone 22 with support sometime soon also coming to flash SailfishOS or Volla OS as well :)"

And here I paste the answer from Samuel (Volla support) in case it might be useful to somebody else.
"Ubuntu Touch is NOT the same as Ubuntu Desktop. The Partition size does differ from device to device (depending on the vendor images that will be used) You will be able to use a 256GB SD card for your secondary OS without any issues. Even 64 would be enough.The speed of your secondary OS depends on the speed of your SD card. However, I can barely see any difference between a system that runs on a SD card compared to Volla OS on the EMMC, even when it comes to launching heavy apps on Ubuntu Touch.You will be able to install Volla OS and Ubuntu Touch via the UBports installer."

@bolly thank you. I'll also write to Volla asking the same question.

@kugiigi Thank you for the answer! I did it following your instructions and it came back!
Yeah Volla 22 has the multiboot option where Volla OS runs in the internal storage and any other OS on the SD.
I'm anyway considering installing only Ubuntu Touch as I'm not a fan of android.
The only thing is that Volla 22 isn't listed among the supported devices but I know you can buy it with Ubuntu Touch installed as only OS plus from Ubuntu desktop there's the option to intall in on Volla 22 from the UB Ports application so I should probably go for it.
I'm very happy to finally have ubuntu on the phone! I've been waiting years for this moment! When Canonical abandoned the project it was a terrible news.

@sven I did install OSM Scout Server under Ubuntu Touch. Volla 22 supports multiboot but as far as I understood in in a week of usage it seems better to update Ubuntu Touch through Volla OS. Before I updated via terminal (on Ubuntu Touch) via apt but I was told it's not supported so better not to do it. Ubuntu Touch runs on an SD card which I bought new of good quality. I partitioned the SD card (256 GB) with 10 GB for the file system and 100 GB for the user system so there's plenty of space.
I hope the description isn't confusing.

I just installed OSM Scout Server on Volla 22 multiboot (Volla OS - Ubuntu Touch) and suddenly the phone rebooted. Back to Ubuntu now there's no App Store anymore. I tried updating it trough Volla OS but nothing changed. Now I'm stucked without App Store. Does anyone know how to fix this issue? Im a bit afraid to try doing "sudo apt-get update" on terminal cause after doing it the system didn't show me the SIM network, i could fix it only by updating through Volla OS.