The overhead to make a cell phone connection is much higher, and requires a point to point connection, thus the network will fail under that load. SMS or text messages don't require the same direct connection but can be bounced about and their lifespan is much greater, thus they can be made when the cell network is unavailable for normal calls. The downside is that text messages can go stale and the protocol does not include reception acknowledgement. Thus you can send 3 messages and get only part of them or receive them out of order. Hardwired phones that aren't VOIP are on a different system, and Satellite phones are a different animal entirely and very likely to function. Your solution of Ham and Marine radio will be a MUCH more reliable alternative to a cell phone or text in the event of an emergency and are quite forward thinking as a survival tool. The only downside to those are the limited people equipped to receive them. I've a neighbor who has both HAM and marine equipment he uses regularly.