I’m not exactly sure what would work in every case, but I’ve managed to do this with both a Google Nexus (the first time) and now with an iPhone 5. Both phones were bought outside of a phone plan and I’ve simply kept the phones in airplane mode, with the WiFi on.
The iPhone required that I get a universal SIM card to activate it (a free/very cheap service), but from that point the phone has worked completely normally in airplane mode.
Some useful apps that have worked incredibly effectively for me throughout this experience are:
- For making free calls over WiFi: Google Hangouts (can dial any number)
- For making calls to your friends over WiFi: Facebook Messenger and Whatsapp
- Google Maps (GPS works offline)