The darkening of the screen and the dropping of the call may be two different issues.
For the screen: check that nothing is obstructing the proximity sensor - no dirt, no fingerprints, no unneeded screen protector.
For the dropped call: check with your mobile operator or service provider; also make sure that your phone has good signal reception. If all is well here, then it may be that the phone of the person you're trying to reach has poor signal reception at the moment of call.
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