Who is 'in the network'?

Now that I have my own cell phone plan (with my long-time carrier, AT&T), I discovered that I am able to view my usage online. One of the more disconcerting things I discovered was that calls to one of my roommates were no longer classified as a 'Mobile-to-Mobile' call - one that doesn't count towards your overall monthly minutes - even though it had been on my family's plan, which I had previously been on. I suppose it's no big deal in the end, but I do think it has probably cut 10-20 minutes off my 450 monthly minutes...which wouldn't be a big deal if I wasn't running up against the limit (thanks to all the calls I had to place to Manhattan real estate brokers). It'd be nice if cell phone companies could clarify who exactly falls under the umbrella of being in-network.