Phone lags and 'crashes'? O.o

[Teezy]

99% of the time when I get a call while the screen is off, the phone rings and vibrates as usual, but the screen doesn't turn on and become reponsive until about 15 seconds later and by then I miss the call. A couple times, the phone app FCs. But after the call "ends, " the device resumes.... not lagging. Any help?

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