But why?
ESN seems like a colossal waste of money, just make some legislation to require all phone companies to make a special SIM that works across all networks and has priority over standard users. It's much easier and cheaper to make it the carriers problem.
And for backup just keep the old system and/or train users how to use regular walkie talkies.
Failing that, why not wait a few years to see what other countries do with their systems and copy them.