If the phones are still ringing that suggests the plane most likely went down over land or extremely shallow water as the radio frequencies used for mobile telecoms can't penetrate it. They would need to be VLF or ELF to manage that.
No...
The ring tone is not generated by the phone - but by the last base station that it 'camped on' to....
So if you smashed your phone to pieces right now there is still a good chance that I could get a ring tone if I dialled it....
It sort of depends on the phone system and software in place....
Just in the same way that if we get a power cut here and all our phones go off (land-lines) you'll still get a ring-tone from the exchange....