Why did my phone start ringing when there is no SIM card installed
That happened to my Samsung Wi-Fi "not smartphone, just touch". 

I live in jordan, and I received a call twice from an unknown number. It rang with no sound on the microphone and no SIM card. It is really weird, but I think the most common factor is the Samsung phone. It might be kind of an emergency call, or the company puts kind of tracking system to see if these devices are still in service.