That all gets back to the "ringing" phones. There should be a phone
company somewhere that can say if they were being pinged by these
phones if they were.
If they were reporting active roaming to a real phone company they
should be able to track that down.
It certainly pops on your bill.
The batteries are long dead by now tho.
They were likely over the open ocean with their phones off or in
airplane mode. There's not a good chance of any mobile phone records
helping.