Vince, I would recommend NOT calling, only because I've seen some people *try* to do what Neon mentioned, and the idiot reps on the other end of the phone canceled the account on them, losing the number.
Honestly, if you bring your bill and account number with you when you pick up the phone, porting in Sprint's system is easy. Toddler level easy. I've never had an issue with a port, other than when someone gave me the wrong account number.
As for getting a temp number and porting later? It's possible, but remember that you WILL need to wipe your phone to get it working "correctly". 9 times out of 10, a simple reprogramming of the phone won't solve all your issues. I personally recommend porting at the time of sale, to avoid any issues.
Former Sprint & AT&T employee
My HeatWare reviews: