As a follow-up...
The other SIMPLE option could be to have different call-in numbers for different functions. By that, I mean one number by which to set and/or change failover, another number by which to set and/or change forwarding. That would require a bit of users setting up their phone to have the numbers in their address book to accomplish tasks for which they feel they may have use. DIDs are cheap! And, the system wouldn't even be restricted to cell phones.
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