[dub24]

*73 for busy, *74 for no answer, or *28 for both. Of course you would follow up with the number you want to forward too (EX: *281234567890).

I tried it today and got an error but then got a communication from Sprint reading this:

Q: I tried to dial *73, *74 or *28 and received an error message.
A: Sprint is currently opening up conditional (no answer/busy) call forwarding for all CDMA customers; however, this customer may not have the changes at the switch completed yet. The target completion date is November 16, 2009. iDEN customers already have the ability to use conditional call forwarding.

So even though its going to be free as of the 8th of November it may not work until mid November for some people :/