I will use the opportunity to remind that Signal is operated by a non-profit in the jurisdiction called "the US". This could have implications.
A somewhat more anarchist option might be TOX. There is no single client, TOX is a protocol, you can choose from half a dozen clients. I personally use qTox.
Upside: no phone number required. No questions asked.
Downside: no servers to store and forward messages. You can talk if both parties are online.