VoIP: Google Voice and OBiTalk make it brain-dead easy
Obihai sells several adapters, such as the OBi100 and OBi110 shown above, along with the OBi202, which bundles dual-port router/bridge facilities. The OBi110 is slightly larger and more expensive than its lower-numbered sibling, but it supports both an Ethernet input (for VoIP) and an optional landline input (for fail-over purposes, in case premises electricity has failed or the Internet connection is down). I own the OBi110, which I purchased a year ago for $49.99 from Amazon.com.
Google Voice is based on two standalone services acquired by Google; GrandCentral, a phone number and voicemail management service purchased in mid-2007, and Gizmo5, a VoIP provider bought in late 2009. Although Gizmo5's SIP-based service particulars had previously been published, thereby enabling owners of generic ATA adapters to tap into them, the standalone service was shut down in early 2011. As such, to the best of my knowledge, Obihai Technology is the only hardware provider that to date has figured out how to add Google Voice capabilities to its products (which, by the way, also support various SIP-based VoIP services).
Setup is straightforward and well documented on the company's website, although it involves sharing your Google account username and password details with Obihai (a fact which, fearing the potential for hacking and subequent identity fraud, compelled me to leverage the Google Voice facilities of a little-used secondary Google account versus those of my primary one). You first activate the adapter by setting up your OBiTalk account, and then punch a specific and service-supplied dial pad button button combination into the handset connected to the adapter.
OBiTalk automatically found the Internet-connected OBi110 adapter and added it to my account; at that point I was able to make free (and so far glitch-free) outgoing calls with no problem. Incoming call attempts conversely still immediately went to Google Voice voicemail. However, the product FAQ straightaway set me straight; one tweak to my Google Voice account forwarding settings was all that it took to get incoming calls working as intended. Domestic Google Voice calls are completely free, as are "soft phone"-based communications between OBi account holders regardless of their locations in the world.
The only qualifier of note is that Google Voice doesn't support emergency 911 calling facilities. Otherwise, between the respective OBiTalk and Google Voice service settings, you've got an extensive suite of customization options at your disposal. One particular recommended tweak that I haven't bothered researching, since my particular router doesn't support user-customizable QoS, involves figuring out the ports and protocols leveraged by Google Voice and OBiTalk as a means of prioritizing VoIP packets over other less time-sensitive LAN and WAN traffic.