I'm a little shaky on the details, but as I understand it 710 is an area code used by approved government agencies, regardless of their geographic location, as part of the Government Emergency Telecommunications Service (GETS). The feds have contracts with the major long distance carriers to process 710 calls. In an emergency, an approved user could, for instance, use a pay phone to call across the country without having to dump a bunch of coins in. The 710 tells the phone service computers that the call will be paid for by the government, and that the call has a high priority. Basically set up so the government has phone/fax usage in an emergency when lines are scarce.
A friend once misdialed my 760 area code as 710, and spoke to a somewhat confused government employee at the other end. After looking up the number she called, I told her to expect the NSA, FBI, and IRS to come knocking on her door at any time.