Facetime email to email

I upgraded my phone and gave my old iPhone 8 to my child. They have their own Apple ID through family sharing. They can call anyone (via Facetime) with a phone number, but she cannot Facetime or message someone in the same scenario (newer iPhone, no phone number, has an Apple ID / email). We're using wifi, and messaging shows up as an option for the person's profile, but messages never go through.

Is there a way to make this happen? Or does at least one party need an actual phone number for Facetime/messaging to work? (The person we're trying to call can also place Facetime calls and send messages, but I don't know that they have done so to anyone without a phone number.)