I read this story in about 2010 or earlier. It is probably much older, as I think it was bundled in a collection (which consisted of standalone-works).
A group of powerful races establishes contact with humanity. The humans can send one person to a trial, where he will be asked questions which will decide something really important (I forgot what exactly, but failure was not an option).
For some reason the scientists assume that the aliens are not able to communicate over interstellar distances in real time. This means wherever the trial is hold (either forgot or is never mentioned), the judges must be there (it later turns out they were holograms, the judges sat at home using FTL-communication). If the human can successfully fake that he is capable of FTL-communication, the trial will be guaranteed to end in his (and humanities) favor.
The judges ask the human some questions which he should answer without help, but thanks to a hidden headphone he can get the answers from the best scientists of earth.
In the end it is revealed that the device is just a recording and not actually capable of transmission. The judges are amused by the try and decide in favor of humanity, though they never tell that they saw through the trick. In the end it is revealed that the aliens are indeed capable of FTL-communication.
Especially the end might be oversimplified. I also can't remember what was on stake, continued existence or approval to join the aliens in their big family.