I generally don't hesitate to ring in on UA flights, and haven't had any issues. However, a couple of bad responses on other flights:
1) LH in Y, FRA-LAX, FA's hadn't been down the aisles in hours, I rang, the FA was irritated and came back and basically threw a water bottle at me.
2) US in Y, YYZ-SAN, FA's hiding behind back curtain on 320 for about 2 hours or so, I ring in and the FA actually gets terse with me for interupting their break...