Assuming you're on the stock Sprint ROM, I was having the same problem until I disabled my device lock (Settings->Lock). Once disabled, my phone stayed on the keypad during calls. If that works for you and you still want to be able to lock your phone, there are other phone locking apps available that will not screw up the keypad (I think S2U2 is one of these). Just do a quick search in this forum.
Edit: Here's the thread with more info on this topic:
http://forum.ppcgeeks.com/showthread.php?t=82944