Your phone number is not your identity in Signal. If you change SIM cards then your friends won't notice and the app works as before.
Signal is based on asym encryption - your private key effectively encodes your identity in combination with your public key.
> And only if the first client is Android?
There is no point in dwelling on that. Android and its syblings are the only potentially secure popular smartphone OSs. Apple and Microsoft OSs are fundamentally flawed in that regard.