I followed the Sprint link from the Columbia J-school blog, and here's what it said about existing customers (which you would be, if you bought the plan from Best Buy first).
Note that it say the discount is for faculty and staff, but here it is:
"For Current Sprint Customers
If you already have Sprint service and would like to receive the 18% Columbia University Faculty & Staff discount, here's what you need to do:
Call (Sprint) 1-866-853-4931 and select option 1 to request your discount.
You will need to provide customer service with the Columbia University Faculty & Staff node number 0080026340.
Every 24 months, eligible Sprint customers can get up to $150.00 off a new Sprint PCS phone. For eligibility, visit http://www.SprintRebates.com
, or call 877-319-6489.
For all billing inquiries and other inquiries on current service, please contact Sprint 888-788-4727."