To prevent the use of mobiles already aboard held by crew and passengers, a couple of mobile phone jammers could have been taken aboard, they are quite effective and some look like wireless access points so could easily be mistaken for a bit of IT kit by airside security.
As has been pointed out already, the pro-mobile brigade are clutching at straws. There could be any number of reasons you could hear a ringback tone on a mobile beyond it just ringing in someone's pocket....
Also, its the 21st century.... its 300 tonnes of aircraft that the whole world is looking for. Even assuming there were jammers galore and the passengers were bound and gagged.... many people outside of that aircraft would have (a) seen it on its way to land (b) seen it on the tarmac. All viable airfields have no doubt been already checked through satellite imagery and local reports. If it landed in once piece, the world would have heard about it by now !
The mobile phone ideas simply have no legs and are simply nothing more than optimistic and hopeful thinking on the part of the unfortunate bereaved families (for whom I have every sympathy and send condolences).