What about phone-pooling? You aren't going to have call all the time, you want only one minute out of ten. So if several phones in one place could arrange it between ourselves: time is separated into small blocks, and during each block only one phone out of these ten tries to connect, others go to offline mode, and tunnel their calls through WiFi/Bluetooth and the phone which is currently online. The problem is that you have to trust other people to tunnel your calls through their mobiles and their calls through your mobile. EDIT: I mean, if cellular towers see ten times less phones in the area, there should be no congestion, and the calls should easily go through.