This brings up more questions:
Actually I wonder how this is going to work?
There is no way that you are going to bring this devise to your face to talk and listen unless you like to attract attention ......

Assumption N°1:
You don't have a phone - you only have access to Wifi.
Then you use the headphones - browse on the net - talk from a 50cm distance - use VOIP. Is this feasible?
The contact App is missing - so you need to call from a kind of Skype with your contact list build into that app.

Assumption N°2:
You have a phone in your pocket and no Wifi.
You connect to the internet trough your phone using GPRS.
You use the same program as with Wifi.
But why not then making a software for the phone and call straight from the phone VOIP.
Of course Mobile operators are going to block this or charge extra. This is competition. But of course landline operators are moving also towards VOIP.
I pay 32 Euro/month flat fee for GPRS on top of other basic phone charges.

Assumption N°3.
You have phone with you and access to Wifi.
You use your phone bluetooth including the phone speaker and mic - Call VOIP and use the tablet to jump on the net.
But then you don't have hands enough..... ;-)

Assumption N° 4
You trash your phone - you always have Wifi ??????? - you use headphones and the build in Mic. (Interesting possibility for people that don't have a computer, but just get a Wifi at home and Hotspot network of the provider in the city)


I could go on with possibities using a bleutooth headset etc....
I am missing pieces in this puzzle..
The mic doesn't make any sense yet for making calls.
But then again why not keeping the thing in front of your mouth......
Any ideas ?