Just to make interesting conversation...

I actually use the nexus 7 as a large SIP phone for making infrequent phone calls. I too only use the phone rarely, so it makes little sense for me to buy a dedicated device. When I take a call, I have to flip the device so that the rear speaker is at my ear. The volume for the device is already set to low levels for phone calls, as it would be on a handset. The only trouble is that I have to be careful not to touch the screen for unwanted input. The setup works well in my home, and when around WiFi.

It would have been ideal to have a dedicated speaker like the galaxy tab 7.7, but honestly, I do not miss it much thanks to this setup.