I usually get this a couple of times a year i have great fun by pretending to be real slow and asking really stupid questions. Ask them to wait while you figure something out put the phone down and do something else for a few minutes then come back and ask them to repeat stuff and then put the phone down etc etc. My record is about 40 minutes and one seriously pissed off guy at the other end before they hang up on me.
It's maybe not their fault they've got a dodgy job though, is it? They're probably earning poor wages whilst being abused by the irate punters they're disturbing. I always try and treat the callers respectfully, whilst trying to cause their employers as much inconvenience as possible.
