While most of the tech world today is reveling in the introduction of Google Maps Navigation for Android 2.0 and its first host device, the Motorola Droid, there's still plenty of Google left to talk about.

Earlier this month, Google Voice was subject to yet another inquiry by the US Federal Communications Commission -- specifically the Wireline Competition Bureau. Its intent is to re-examine what Google Voice exactly is, evaluate whether it is unfairly blocking certain connections and how, and to determine how it should be regulated.

Today was the deadline for that inquiry, and Google responded with a 12-page letter responding to the five principal questions posed by the FCC. Two of the responses were kept confidential; they dealt with the total number of Google Voice users, and the third-party partnerships Google has made for providing access to telephone numbers, transmission of calls, and interconnection with local networks. Fortunately, the heart of Google's response to the call blocking allegation was left intact.

The FCC's key question was three-fold: "How does Google identify the telephone numbers to which it restricts calls? Does it restrict calls to individual telephone numbers, or to particular exchanges or NPA-NXXs? Why does Google Voice restrict these numbers?"

The simplest and most obvious answer to this last question is: cost.

"As a free service, Google Voice is predicated on the ability to manage operating costs aggressively. The vast majority of outbound calls to phone numbers in the United States can be terminated at reasonable costs, which allows Google Voice to provide free forwarding to US numbers and free calls to those numbers," Google's response began.

"However, through the application of a set of data filters, we have found that calls to a relatively small number of telephone numbers generate vastly disproportionate costs," the letter continued. "These telephone numbers terminate to local exchange carriers located in comparatively high cost destinations, which in turn have set up various businesses to encourage inbound calling, that apparently include conference calling services and chat lines. If Google Voice were required to terminate calls to those specific numbers, and spread the costs among its user base, the free service model for all users could be jeopardized."

In Google's investigation, it found that the top 10 prefixes to US destinations (NPA-NXX) accounted for 1.1% of monthly traffic by volume (161 times higher than the expected amount per prefix), which meant this tiny number of prefixes accounted for 26.2% of all of Google Voice's monthly cost. Google said its own underlying carriers assessed Google Voice calls to these numbers with these prefixes to be as much as 39¢ per minute and therefore restricted them.

Google goes on to say that it originally could only block by prefix, but now has the technology to block by the full telephone number, and the list of restricted numbers amounts to fewer than 100.

AT&T's senior vice president for federal regulations Robert Quinn said that Google Voice's blockage of such numbers is a luxury not afforded to its "competitors" the telephone companies, and is therefore a transgression of the Internet Policy Statement. However, it has not yet been determined if Google Voice is in fact in competition with the telephone companies, and further, as a free service, if it would be subject to the same regulations as subscription-based services.