Ask HN: How are free phone call apps getting their access?

Surely they're not paying twilio or another similar company $0.01 per minute to allow you to make and receive phone calls... nor are they paying $1/month for a phone number that they give away for free.

How are apps like fongo and magicjack providing free phone numbers and free phone calling?

I know they are ad supported, but I highly doubt they're making over $1/user/month with those advertisements.

Or are they?

Thanks.