As Captain Obvious would be wont to point out, tablets aren’t really made for making phone calls. This, however, doesn’t mean that you can’t use your tablet to make phone calls. In some cases, it can even be preferable. Google Nexus 7 owners can now get in on the phone calling action. It requires that you have a Google Voice account to make phone calls from, but otherwise the mod isn’t all that difficult.
Released by XDA Senior Member bongostl, the guide instructs users on how to modify the Nexus 7′s framework-res.apk using a tool provided in thread. Some users running custom ROMs have reporting boot loops, but bongostl is trying to help users by modding their specific files personally. So for now, it only seems to work on the stock ROM.