You know what would be really cool, is if Signal supported encryption over SMS. Then you could communicate without the cooperation of either Google or Signal's own servers! And no battery life impact!
...except it did. And they killed that feature, justifying it with a bunch of really weird arguments [1]. Like that SMS always leaks metadata, when Signal's data protocol also leaked the same metadata - except only to them, instead of a federated network. But don't worry, they promise they don't log it. Honest.
I don't trust Signal at all after that. Use SilenceIM for encrypted messaging [2].
[1] https://signal.org/blog/goodbye-encrypted-sms/ [2] https://silence.im/
Note that while this technically isn't the case any more, as you can use Signal without Google services now, Signal still creates a background task which periodically checks for updates and you can't change the frequency. This results in the GMS-free version to be a major battery drain [1]. Ultimately, this was a deal breaker for me and I removed Signal after testing it for a while on my smartphone.
[1]: https://github.com/signalapp/Signal-Android/issues/6732