Saying press the call bell is all well and good, but in my experience it's often ignored. I was on a CE flight recently, pressed the call bell as we wanted a top up and there was no sign of any cabin crew. After a while, someone emerged, took no notice of my call bell and headed to the back of the CE cabin to offer the people there more drinks - we managed to flag her down to get some too, but there was still no acknowledgement of the call bell.