In principle, since both the Signal client and server are FLOSS, it should be possible to resist a block in some ways that might not be so easy with for example Whatsapp, but as far as I can tell, as things stand right now there is no built-in way to switch server, and asking millions of regular people to make changes to the source code of their cellphone software, recompile and manually reinstall does not seem like a recipe for success. I wonder if one might sensibly work around these things by fallback/optional connection to the server through Tor if Tor is available on the device?