You can always count on the xda dev community to come up with ways to tinker and toy with things that were never possible before. Thanks to XDA Senior Member bongosti it is now possible to use your Nexus 7 tablet as a fully functional phone, as long as you have a Google Voice account.
For now the mod mostly only works on stock ROMS, since many custom ROM users are experiencing bootloops. However, bongosti is working hard to work on a case-by-case basis with many people to make it work.
The process doesn't seem like it'd be too difficult for beginners to android modding since there are only a few, relatively straight-forward steps. The instructions are available here, bu for the downloads, and more in-depth instructions hit the jump at the bottom of the page to the original thread. Again, a huge thanks to bongosti over at XDA.
Step one: configure incoming:
1) set up a free app account (i use callcentric)
2) setup a free incoming phone number (i use ipkall.com). (note:server is in.callcentric.com) to connect to your sip account
3) setup a sip app on your tablet (i like csipsimple.... Free in the play store) and connect it to your callcentric, account (user name is your callcentric number... Should start with 1777)
4) go to Google voice on your computer, go to settings, and add a new number. Add the number you registered with ipkall. It will call and confirm you. It should ring through to your tablet! You now will receive incoming calls to your tablet!
Now, let's get you set up for outgoing calls.
1) download voice+ from the play store and connect it to your Google voice account.
2) download exdialer from the play store (dialer one doesn't work... I don't know why)
Now what you need to do is modify your framework-res.apk to make the tablet think that it is voice capable so that voice+ will be triggered when you make a call.
so, here is what you need to do:
1) download the framework-res editor that is attached here and unzip it
2) copy your framework-res.apk from your ROM (found in the /system/framework folder) and put it in the 'place-framework-res-here' folder
3) double click on 'run me' and recompile your framework-res
4) in the 'files-to-edit' folder, click on framework-res.apk>res>values>bools (edit the bools.xml file)
5) find the line false and change 'false' to 'true'. save and close the file
6) in the framework Flasher that you opened in step 3, choose '2' and compile a new apk
7) choose option '3' and sign the apk.
8) choose option 4 and create an update.zip
9)flash the update.zip that you made