It sounds like 78990 is the number we're looking for.
If you call it it should ring the receptionist phone. You didn't say that specifically but that's what I read into it.
In programming look for the form: Call Forwarding Profile
Add an entry for 78990, call type Always, Destination 7790.
Turn it "on".
As long as the receptionist at site two can call site one by dialing 7790 then this should work for you.
Turn it off when you don't want it forwarded..
Ralph