What is it with people (like the sales girl in the office opposite me) who tell the person on the phone to hang on while they answer their other phone, and answer that with, "Hi, can I call you back? I'm on the other line"???

Surely that's what voicemail is for? It seems that doing the above is impolite to both parties (the original guy you were talking to and the guy you picked up just so you could cut 'em off).