Very ex-ba -
The max radius of a cell in the GSM system is 35KM, usually much less. CDMA is about double that in theory (although in practice, about the same).
If the postulation is that the flight had descended to low level, then those ranges hold true. They would have to be within 35KM or so of a suitable cell. Added to the attenuation effect of being inside the aircraft skin, it doesn't seem likely to me. If the aircraft landed on a remote airfield (also seems improbable to me), it would have to be so isolated as to be entirely devoid of communications for it to have not been picked up by now. You can't hide an intact B777 when the world is looking for you. There's always the sat phones of course, but they aren't exactly commonly carried.
On the subject of the those whose phones have been called and they rung, this seems like rubbish to me. For the phone to have appeared to ring, the phone must be registered with the network (i.e. in a group of cells), been paged, and then responded to the page. If that had occurred, the network KNOWS which cell the mobile responded from, and records it. Ergo, the SAR authorities would know where to look. If they knew which cell to look in, I doubt it would take so many days to find it, and I would have expected the authorities to have said they know where the phones are/were.
I've had nothing to add on the aviation aspects, I'm just frequent SLF, so I have kept out of that, but I am a telecom engineer.