To add to the recent postings from Telecoms people -I am not a mobile phone expert but I do know about undersea cables and one of the big problems here is elapsed time since the incident if there are significant surface or sub surface currents in the area.
Even a modest surface current of 3 or three knots can expand the potential search area by an enormous amount after three or four days creating a classic catch-22 where the longer it takes to find a trace means the harder it is to find a trace.
Sub surface currents are not always well documented and do not necessarily act in the same way as surface ones. So rather than a predictable drift of say 2 knots eastwards they can add a sort of crosswind component effect creating a north or southeast vector making the SAR job harder still.
Otherwise it seemed very unlikely from the start that there is, whatever the cause, going to be a very sad outcome