Well he should not have been fired. He should have got a warning not fired.
why should he even be warned? He should be rewarded for not abusing the caller back and merely hanging up.
Discrimination on the basis of race or religion is illegal in the USA. The correct thing to do here is to trace the caller and fine him/her for practicing religious discrimination.
And if this was the reason Sprint fired this guy, shame on them and they should be sued