Google Voice (ex GrandCentral) is definitely another unified communication solution available in the market. As Google seems to have already planned, the claimed telephony service will be soon opened to everyone, publicly. The basic concept is the same developed by IBM through its Sametime Unified Telephony, in which a single unified number is assigned to each user and then associated to a set of personal devices by setting simple routing rules. Just the implementation is what deeply differentiates the two solutions. Google is for now interested in opening this service to a small group of test users. To get an invitation try to apply through this website. Good luck!!