That very well could be, as I mentioned earlier, you might get yourself a VPS and set up an SSH tunnel or something like that, but otherwise I have no idea.
I do know that you can use signal over tor because I do so, but I only use it for the sending messages and voice messages part. As I said, I think the audio call uses UDP and tor is TCP only to the best of my knowledge.
The only other thing I could think of would be to try a mix net such as Nym.