XDA member bongostl has posted a step-by-step guide for modifying your Nexus 7, or any tablet for that matter, to enable outgoing and incoming calls through Google Voice. This requires the editing of system files, so your tablet needs to be rooted. What this method does is make the tablet think it’s voice capable, which allows dialer app voice+ to connect through Google Voice and place a call.
Incoming calls will be handled by any SIP app, such as CSipSimple, which requires you to also set up a call number and routing using other online services such as callcentric and ipkall. Couple all that with Google Voice, an edited and recompiled framework-res.apk file, and the flashing of an update.zip, and you’ve got yourself a really large tablet-phone in 16 quick and easy steps.
Or you can download GrooVe IP from the Play Store and skip all the rest.
Ok, that was snarky. To be fair, bongostl’s method has the advantage that you can still receive calls in your Gmail on your computer, whereas with GrooVe IP, it’s one or the other. But seriously, to me this all seems like way too much of a hassle with too many potential points of failure to really be worth it for most people.
Bottom line: Hackers and tweakers only. Everyone else just get GrooVe IP.