I doubt this is at all likely, but they could conceivably have used those increasingly endangered commodities, public pay phones, by prearrangement. If they had the numbers written down and had determined times in advance to check in with each other, it would be extremely difficult to track any communications back to them. Best sites to use would be ones with banks of phones where usage is heavy and people come and go -- bus stations, airport, motel lobbies, etc.
I think that probably takes more organizational skills than these people have so far demonstrated, but it *is* possible....