If you don’t pick up when I call, please call me back. And don’t tell me why you didn’t pick up. It’s perfectly legitimate for you not to interrupt what you’re doing every time I ring, but the odds are good that I’ll just feel offended that [fill in reason here] was more important to you than I was. So, just be classy, call back when you can, acknowledge that you saw that I called, and let’s get on with things.

[This is why I so prefer email and SMS to phone calls most of the time. Ah, the perils of synchronous communication.]