I know you said you had call waiting, and that you were expecting some important calls, so you had reason to answer.
We have an answering machine on our land line which announces who's calling, and where we can hear any message being left. I no longer answer that line unless it's from someone I know.
I do more or less the same thing with my cell phone. If it's someone in my contacts, I'll answers; if all I get is the number, I often don't. (though I do have to remember that my doctor's number is restricted, since it's part of a big HMO, so if I'm expecting a call from the doctor, I do answer other numbers.)
Other than that, "I'm sorry. This isn't a good time. Goodbye", and hang up.