This story most likely dates from the 1980s, or possibly the very early 1990s.
The protagonist has discovered a BBS that you can download cool stuff from (like most BBSes) as long as you first give it a unique fact first. (I believe it was unique to you as opposed to globally unique.)
The story was set in the US and one of the odd things that catches the protagonist's attention is that it is using an unassigned area code. Upon investigation (phreaking, maybe) the phone number is traced to a building that turns out to contain some highly advanced alien communication gear.
The next fact the protagonist enters is something like "the owner of this BBS is not on Earth." He is congratulated, and promoted to the next level. There is some concern if the aliens are going to start demanding sensitive information about Earth; instead they start asking questions about culture and music.
I was sure this was by Vinge, but I just checked through True Names... and Threats... and it's not there. Which means that I have no idea where I read this; it's even possible it was something I read on Usenet.