Yarp. Signal has a handy invite contacts thingy too. And you can do video chat like FaceTime.

Yes. I don’t really use more than 120 min per month. Most of the folks I call use Signal for voice.

ninja edit: jmp customer service has been phenomenal

I have done this successfully with JMP. Started with a test number that I still use as a burner for online services that require a phone number. It worked well enough that I ported my main cell # over.