There are time that I'll get a phone call, I'll go to answer it, slide my finger across, then it freezes, continues to vibrate, but won't let me answer the call. I have to hard restart the phone in order to use it again. It only does this occasionally, but it can be quite an annoyance, especially if it's a business call.
It happens to me as well, but the call always picks up eventually and I've never had to reset because of it. Just be patient and if it's really a bother, you can always restore.
Maybe just cut back on some of the stuff you're running like Poseidon said.