According to AT&T, Google isn't playing by the rules with Google Voice -- they are rejecting outbound calls to some rural areas to reduce their access expenses. Google says that these numbers are typically used for things like sex chat lines.
Why would it be against the rules if Google decides to block numbers like this? Well, carriers are required to put through calls to anywhere (regardless of cost), and AT&T thinks Google Voice should have to follow the same rules.
Google argues that they aren't a carrier, and don't have to abide by the same laws. Here's how Google differentiates themselves:
This battle between Google, AT&T and Apple (over the Google Voice app) is definitely heating up, and will be fun to watch.