While on a call

test your prox sensor (sensor that sees if the phone is being held up to your head and shuts off the screen so the jumping doesnt happen)
make an arbitrary call to a friends phone but dont put it up to your head.
after teh call connects put your thumb over the sensor(just left of where it says 'sprint' at the top of the phone)
make sure the screen goes black
if you remove your thumb the screen should come back
do this a couple times
the screen should go black and return within 1 second (maybe 2) each time you do this test
if it doesn't you should either reflash 2.1 or take it to sprint for warrenty repair or replacement