This is a really interesting and logical move for FB, to integrate as a tangential add-on with something transactional like phone calls, (instead of just being a destination website / app).
Especially now that WhatsApp (recently acquired by Facebook) tries to hijack voice calling already. Even if you press the phone number in the WhatsApp contact details.
This was a major annoyance for me because I was used to abusing WhatsApp as my address book and the voice call quality with WhatsApp is often atrocious (plus it uses up my phone's very limited data plan).
