Sim boxes and etc aren't useful for terrorism, or at least you not anywhere near this many.
You only need this many for bulk messaging/calling. Legitimate bulk messaging/calling would be going through sip providers and SMS aggregators and/or interconnection with carriers at the kind of volumes you'd have this many sim boxes for. So it's got to be fraud/abuse of some sort. Probably selling bulk sms/calling to users that can't or don't want to use legitimate providers.
Again, I don’t know what the Secret Service isn’t saying, but I do know that failures of imagination have led to unpleasant surprises in the past. I’ll be very curious to learn more. Hopefully the details will come out.
You only need this many for bulk messaging/calling. Legitimate bulk messaging/calling would be going through sip providers and SMS aggregators and/or interconnection with carriers at the kind of volumes you'd have this many sim boxes for. So it's got to be fraud/abuse of some sort. Probably selling bulk sms/calling to users that can't or don't want to use legitimate providers.