I have tried all the suggestions to get SOS off, but nothing works. What should I do now?

Have you contacted your carrier and checked that everything is OK with your cellular plan, and that you are using the phone in a covered area?

if you have done this and still cannot pick up a cellular signal, your phone might have a hardware problem with its cellular radio. In which case you may want to contact Aople or an Apple Authorized repair shop about checking and possibly repairing, the phone.