Try asking THEM to hold while you get yourself a drink or something. It's not so nice for them then! Agree about avoiding premium lines where possible, I hate calling those.
I actually went through a phase in my teenage years of singing to people on the phone when they were cold calling. I always thought of the songs at random so they were quite funny. The best one was when I started singing Amazing Grace, the lady on the other end joined in, and then at the end said "God bless you, child" and it sounded as though she was crying. Oops!