Perhapshas some background in phone phreaking?
In the midwest, as I recall, it used to be that dialing "575" + the last 4 digits or full phone number (can't remember which) and then hanging up would send a "ringback" to your phone that would prove the line was working… If you wanted to prank your friend, you could just tell them a cute girl from school said she was gonna call him, and then do the 575 trick every so often (when you go to the bathroom, etc), and watch him leap for the phone, only to find clean dialtone when he picked up.
Either that, or he plans to move to New Mexico after the IPO (they recently added the 575 area code somewhere out there).
He could also be referring to "5×5" – meaning perfect signal strength and clarity, and of course the "perfect number 7"… Could it be some kind of reference to perfectionist traits of the Groupon CEO?
Finally, the most likely option is simply that he's a fan of "Seventeen" magazine.