Phone number Facetime doesn't work!

Hello everybody!

I tried to Facetime a friend of mine, but it said she wasn't available for Facetime. She said friends of her were able to call her, but only when using her e-mail.
When I asked her why I couldn't call her by her number she couldn't tell. So I tried to solve the problem, but didn't succeed. She sees her number, but she can't select it to be reached by it. Is there any way to solve this problem?

Thanks!