I could not restore the original firmware, now I keep it, because I managed to do the configuration and..... go.
At the beginning of this 3d, you can read who did the same thing you want to do too. However the difficulties are not few, and it seems that you have to insert some command lines manually to avoid that the VTO closes the call. I do not go further in this explanation because I am a rookie, and I do not know how to do it, for this reason I have a different solution.
The VTO is like an IP phone, so you should be able to record it on any SIP server even with the original firmware.
The important thing is to make the correct configuration in the menu (Device manager) otherwise when you press the button the call is not forwarded correctly.
I had also tried to put the Asterisk server on the raspberry, but I couldn't connect the VTO because I didn't know Asterisk well and I was wrong about the configuration. Note: the menus you find on the VTO are different from those of Asterisk and different from Frit!Box this has sent me in big confusion
I advise you to use EyeBeam on a PC to configure Asterisk and then copy the configuration to the VTO, because every time you need to change parameters on the VTO you have to restart it! And that makes you waste a lot of time.