What is the country code for dialing DSN back to the US?

None. DSN does not utilize country codes. All DSN numbers are 10 digits and permit dialing of 7 digits if in the same "area code". There are 140 "area codes" worldwide. Six of these are for geographical representation and the two that might be what you are looking for are: 312 for CONUS and Puerto Rico as well as 317 for Alaska.