The Fortnite community are trying to solve the Durrr Burger mystery with many visiting the area and asking the agents different questions. They do keep stating to keep trying the number that has been handed out on the card and it seems as though new audio can be heard, by pressing 06 as an extension. This was uncovered by @FNLeak and you can hear the audio below:

By looking at the audio as a spectrogram, the following is revealed:

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New audio 2 Spectrogram

We can see that the numbers 48.8559485 & 2.3611878 have been uncovered. These numbers are speculated to be co-ordinates and when searching these in Google Maps, the following location shows up:

Fortnite Map Location Coordinates

The sign on the building says “Centre de recherches historiques et juridiques”, which according to Google Translation, translates to “Historical and legal research center”. There has not been any reports of players visiting this location yet, but we will keep you updated if there is any more news.